SET amp comparable to First Watt SIT 1?

We don't have enough info from the OP wrt room size/characteristics and which model Volti he's considering, to answer definitively. Also, I can't find an impedance spec on the Volti site. Nevertheless, I have owned OTL amps and like many of their characteristics, but I've never heard one, especially using massed output tubes, that has the sheer transparency, speed, holistic tone and spatial precision of Audion SET overall, and especially the Golden Dream monoblocks, which would only require a little stretching of the OP's stated budget. I think at ~100db efficiency, 24w with smooth clipping should be plenty, unless the room is cavernous.

I also think at a step-down price of $10K/pair, the Audion Golden Night @10w has most of the qualitative character of the Golden Dream, minus some of the shove. With KR tubes, the bass will gain some discipline. Having recently gotten some experience with the Takatsuki 300B, which is very good, I'll still take the KR balloon glass full range, and it's half the price. Equally well made.

Charles, you know I use Zu speakers at 101db/w/m, and that I've written elsewhere that while such a speaker can clearly be paired with some of the beautiful low-watt 2a3 and 45 offerings, I think they give up too much shove and dynamic fidelity. When 300B is done as well as Audion implements it, and there's a PSET option, why cramp yourself? For shove over ultimate nuance and transparency, the 845 Black Shadow is squarely in the OP's stated economics too, and would put a firmer grip on the very bottom end. But overall, for what the OP seems to be after, he can pretty much index his preferred price to the Audion SET offerings and go, knowing that for the dollar level chosen the march to the Volti will be sublime.

Phil

A bit different recommendation: Audiopax Model 88.

I am coming from an Audio Note Quest Silver and it has everything the Note had plus more. It is capable to show the same midrange magic but has much more versatility because of its power reserves. I am using it with Avantgarde horns where it shouldn't matter, but it does.

It is however a rare find these days (sadly).

Phil,
I've used the Takatsuki, KR and EML XLS 300b in my Coincident SET amplifier and all three are of premium quality level. I have a slight preference of the EML XLS  over the KR but could reflect amplifier differences as well as listener.  Border Patrol vs Audition is an  interesting contrast as their approach is quite  different.  The BP utilize large external power supplies whereas Audions are smaller, more compact and noticeably lighter in weight. Both have earned esteemed reputations and offer upgraded models in their lines. As said before, there are numerous top quality choices among the 300b SET amplifiers.
Charles 

Audiopax Model 88 utilize the KT 88 tubes and is an exotic and more complicated design than the very simple circuit of a 300b SET amplifier. The Audiopax was very well regarded but its talented Brazilian designer/builder died several years ago. This would be a rare find on the used market.
Charles

Is an SET design really necessary? I have a VAC amp that uses 2 300b's per channel, and it drives just about anything. I had it on a pair of ESL's for over 2 years and had absolutely no problems.