IME pentodes won't make much of a difference and in fact I prefer triodes for a lot of this low power stuff. They are smoother sounding and easier to deal with. I think part of the secret of "big sound" is to get enough air moving with the speaker. In general, a speaker is the one component that will have the biggest impact on the sound of an amp anyway. This contradicts the idea of a "low volume" amp somewhat, but in the end I think it's a balance- getting "enough" power to make the speaker sound right and sound "big" without requiring so much that it causes hearing damage.
I have never heard a nano clip that I thought sounded good. But I will reserve judgement on the nano because that could be due to the clips and I have not heard one in person. But IMO it is easy to build something that buzzes and has enough power to drive a speaker. Does it sound "big" or "good" or whatever, though, that's a different story.
To be honest, I think a dummy load, line out, speaker sim, mixer and headphones can sound better than some small amps I've heard. If you are interested in low volume apartment level don't wake the baby stuff, I think that approach can be more effective- even a POD or something (perish the thought
I just don't hear "big sound" through a speaker below a certain threshold of minimal volume, but that's just my experience. YMMV...