SS DIY headphone amp to build... after SOHAII, SSMH, Mini3 and Gainclone

Looking for a new build, something higher-end and SS... started my journey with a iMod mini, CMOY, SSMH, Gainclone, Mini3, Gamma1 and a SOHAII.

At the time I was using HD580s, which have since been sold and will be replaced with HD600/HD650/HD6xx one of them. I'm also interested in planars, maybe the Sundara.

So I haven't built an OTL or higher-end SS headphone amp. Considering building the latter and I wanted some ideas on what I should pursue, given what I've built and hence something "technically" better.

So thinking of builds based on some perusing:
  • O2
  • MilletMax
  • Whammy
  • M3
  • Beta22 (2-channel)

I'd like to keep the cost sub $500, though I'd have some room for something exceptional. Please, open to suggestions.

On a tangent, I hear Bottlehead Crack or SEX mentioned a lot for OTL... thoughts?
 
I built the AMB Labs M3 or called M cubed headphone amp. Easy build and very clean sound. I have AKG K 702's and work great with them. I also have built O2 and Milletmax which are very good. I also got a Headphone amp QRV-01 from Sjostrom Audio which has very good specs that I have not built yet. He has a phono preamp QSX Mark II that i bought that has a crazy amount of transistors that i will build soon.