Zaph ZRT 2.5 sealed opinions?

Have you tried EQ on the sealed enclosure? It should respond pretty well. It might make you feel better knowing the drivers are capable of the bass you want. It's not very clear what you're after though. Do you just want more bass than the 5" Revelator or are you expecting the room to shake?

Also, before you build a 68L enclosure, what about buying or building the 2 way crossover and plugging the lower woofer hole? The two way design should have more bass output than the ZD5.

I keep reading this thread because I have a similar build right now. Mine is the same 18W8531G00 woofer in 33L @35Hz as I posted before. The bass output is definitely more than my build with a 15W4531G00 in ~9.5L @ 50hz which isn't too far from the ZD5.
Thanks for that, that was exactly my next question - would a 2-way actually reach deeper in low end than this 2.5version and ZD5 (I don't need more bass, I need it to go DEEPER)? The reason I didn't go for 65L in the first place is that my "room" is small (11m2).

In short, I am not after bass volume, I am after low end depth, to run flat as deep as possible.

@D1sco - they are in phase - checked, doublechecked and triplechecked with 15Hz signal generator from REW. There's plenty of bass, it just does not reach deep enough.
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I completely forgot one detail about ZD5 - it was built as 25l floorstander, but with reduced depth minus the sand chamber. This was messing up the mids, so the box was eventually reduced internally to 14L, but the vent remained the same - it was tuned to 30Hz for 25L box (3,8mmX123mm). With reduced volume according to the Virtuix, the tuning actually went up to 38,4Hz. So I ended up with a unique sounding 14L vented ZD5 that noone has :) I will try to tune ZRT to the same frequency so maybe it will be able to match that low end reach that my ZD5 has.

I'm wondering whether Zd5 tuning and placement just happens to align to your room modes. Doing the same to the ZRT might net the bass response you're looking for.
Personally I would also try stepping through sine wave increments (WinISD and REW have generators) and just listen to ZD5 taking notes as you step half octaves up the scale. Repeat with the ZRT and compare.
So the conclusion is - nothing wrong with ZRT, I just chose the slightly wrong design. They are high efficiency speakers that don't go very deep. The voices and pretty much everything sounds just incredible, beats B&W 703 S3 on almost everything. I have my eye now on DXT MON 182 ( which looks impressive on low end. If they can match voices on ZRT, that would be a way to go for me. Low efficiency does not bother me, I run on class D or AB. Thanks for the help everyone, time to close this off :)