Just to start...depending on how you build Dario's My_ref EVO boards (which opamps you choose to use, which specialized resistors/aps you choose in key positions and which signal-entry cap you choose) you get a dramatically different sounding amp. Same with the ACA's behavior although one is really limited to stock parts or the variation by tungstenaudio (the way I built it which sound really nice).
My everyday favorite is Wayne's BA2018 (I've built two:compact & fuller size chassis) which are driven by either Pete Millett's phono preamp's PS (http://www.pmillett.com/LR_phono.html) or my main one using avdesignguru's PS (https://www.diyaudio.com/community/threads/waynes-ba-2018-linestage.329240/page-135 ; post 2696) but using Sparkos Regulators. The BA2018 is a clean, fast, precise sounding amp with good bottom response, non-fatiguing high frequency reproduction and a wonderful airiness to it. But, if you have a speaker that is bright you may not like it. The BA2018, with the ACA, single or mono-blocks, is a nice match. It adds a bit of upper end to the ACAs mid frequency warmth. It's my standard preamp with how I built Dario's amp.
Pass' BA3 is a neutral-to-slightly warm sounding pre-amp that can be adjusted to slightly change its sound but you'd need an oscilloscope to really adjust it for accurate changes in harmonics (you can do it by ear if you want to try). It's a bit tricky to adjust it because any breeze over the jfets (changing their temperatures after warmup) will make dc offset and bias adjustments slightly difficult to get spot on (if you really have to is another question). It's really not great if you want a sparkling high end but probably great for jazz vocals or want a warm and fuzzy type of sound. I like it with most of the inexpensive class d amps I've put together but not with Dario's or the ACA amp. Just me though. It needs a really clean PS to sound it's best. I think I used the avdesignguru's PS with Sparkos Regulators again as in the one BA2018 version above.
The Korg B1K amp (from Pass as well) is an interesting preamp. If you want real mid-frequency warmth added to either Dario's amp or the ACA this is a nice unit. It gives a nice soundstage but it lacks the nice, breathtaking upper range that you can get with the BA2018. Again, maybe the 2018 is great/best for my speakers but not others. If you listen to a lot of Jazz vocals this preamp is nice. Is it really "tube-like"? I can't answer that since I've never had a real tube preamp. I also use a good external PS for it (the AMB.org Sigma 11). Using a wallwart really doesn't do it justice.
ZenMods Iron Pre (SE version) is a very clean, precise preamp and it does sound well with the ACA but I don't think it compliments Dario's amp well. The Iron Pre has a really solid bottom, smooth middle & upper range but doesn't give me the nice, airy highs that the BA2018 does. But, if you prefer a preamp that gets out of the way of the music and you already have a really nice sounding & powerful Class A or high current amp this might be the preamp for you (just my assessment). It works well with the ACAs but doesn't please me with Dario's amp, at least the way I built the My_ref EVO amp.
As far as the older Yamaha or ADCOM preamps go they sound nice as well and have the advantage of frequency adjustments and phono preamps built in (if you still use a turntable). When set to flat they are just ok sounding but nothing to dismiss as "bad sounding."
Cheers...hope I spelled everything correctly...
Pete
My everyday favorite is Wayne's BA2018 (I've built two:compact & fuller size chassis) which are driven by either Pete Millett's phono preamp's PS (http://www.pmillett.com/LR_phono.html) or my main one using avdesignguru's PS (https://www.diyaudio.com/community/threads/waynes-ba-2018-linestage.329240/page-135 ; post 2696) but using Sparkos Regulators. The BA2018 is a clean, fast, precise sounding amp with good bottom response, non-fatiguing high frequency reproduction and a wonderful airiness to it. But, if you have a speaker that is bright you may not like it. The BA2018, with the ACA, single or mono-blocks, is a nice match. It adds a bit of upper end to the ACAs mid frequency warmth. It's my standard preamp with how I built Dario's amp.
Pass' BA3 is a neutral-to-slightly warm sounding pre-amp that can be adjusted to slightly change its sound but you'd need an oscilloscope to really adjust it for accurate changes in harmonics (you can do it by ear if you want to try). It's a bit tricky to adjust it because any breeze over the jfets (changing their temperatures after warmup) will make dc offset and bias adjustments slightly difficult to get spot on (if you really have to is another question). It's really not great if you want a sparkling high end but probably great for jazz vocals or want a warm and fuzzy type of sound. I like it with most of the inexpensive class d amps I've put together but not with Dario's or the ACA amp. Just me though. It needs a really clean PS to sound it's best. I think I used the avdesignguru's PS with Sparkos Regulators again as in the one BA2018 version above.
The Korg B1K amp (from Pass as well) is an interesting preamp. If you want real mid-frequency warmth added to either Dario's amp or the ACA this is a nice unit. It gives a nice soundstage but it lacks the nice, breathtaking upper range that you can get with the BA2018. Again, maybe the 2018 is great/best for my speakers but not others. If you listen to a lot of Jazz vocals this preamp is nice. Is it really "tube-like"? I can't answer that since I've never had a real tube preamp. I also use a good external PS for it (the AMB.org Sigma 11). Using a wallwart really doesn't do it justice.
ZenMods Iron Pre (SE version) is a very clean, precise preamp and it does sound well with the ACA but I don't think it compliments Dario's amp well. The Iron Pre has a really solid bottom, smooth middle & upper range but doesn't give me the nice, airy highs that the BA2018 does. But, if you prefer a preamp that gets out of the way of the music and you already have a really nice sounding & powerful Class A or high current amp this might be the preamp for you (just my assessment). It works well with the ACAs but doesn't please me with Dario's amp, at least the way I built the My_ref EVO amp.
As far as the older Yamaha or ADCOM preamps go they sound nice as well and have the advantage of frequency adjustments and phono preamps built in (if you still use a turntable). When set to flat they are just ok sounding but nothing to dismiss as "bad sounding."
Cheers...hope I spelled everything correctly...
Pete
Thanks again. Currently I've just been using my NAD C320Bee integrated preamp and bypassing the power amp. I wish my Sansui AU-719 had preamp outs.