what is the best se tube amp

Single-ended tube amps are a fairly large category. It will depend on how much power you need. For your Proac 2.5 speakers, you'll probably need at least 10 watts, and probably more.

I think the best(lower power) SET amp made is a SET-ZOTL from David Berning called the Siegfried. It can be ordered in either 300B(8watts) or 811(11 watts). This amp has no traditional output transformer, which can be the "achilles heel" of a SET amp. The speed, openness, transparency, and detail are amazing. My personal amp is a custom-made variation of this, using Type 45 triodes with choke-loading and battery power. I can vouch for the excellent sound.

For the higher power levels, the Antique Sound Lab AQ1009, and Viva Aurora sound very nice. If you want much higher power levels, WAVAC produces some 100 watt, or even 150 watt SET amplifiers that can be pretty expensive, but excellent.

Also, the kind of triodes used(300B,811,845,833, etc) are very significant in the kind of sound that the amp produces. So is the brand of tubes that are used. And the circuit design.

You should do some research into this subject yourself. All SET amps don't sound the same. My recommendation is to get a highly regarded unit with the power levels that are suitable for your speakers, and make sure you get the best output transformers offered, even if you have to special order it and pay more for the transformers. In most cases, NOS tubes will also pay off.

Lamm's

David Berning Seigfried - stunning.

Oh brother....another, "..what is the BEST" query! It's like
asking a bunch of contractors "What is the best hammer to
buy?" Or in this case, since the options are far more broad in their
potential for investment, asking a great variety of people, "What is the
best sports car to buy?"

That said, Tom's (Twl) answer is informative and to the point. They all
sound quite different, and pairing them off with the speakers and
preamp is critical as well. A poor combination there could make you
swear off SET forever. They all offer varying degrees of strengths and
weaknesses of different qualities. Personal preference for exactly which
ones are a priority is paramount here. I have no idea how your ProAc's
pairs off with with SE amps but a search may yield some insight there.
In general you will need efficient speakers on the order of 90db or
greater to go with the low-powered options.

I'd also give Boa2's recommendation of George Wright's SET products an
enthusiastic thumb's up; both the 3.5 and his Mono 8 offer tremendous
bang-for-buck in a very no-frills, function-over-form cosmetic package.
Go over to AudioAsylum and check in the SETAsylum there for further
input on this subject.

No doubt, Berning's and Lamm offer stunning products....you may also
consider Border Patrol and Electronluv if you are looking in that price
range. But you should also know that those recommendations (since
you left your query wide open) are about 8-10 times the used price of
your CJ Premier. Given that fact I'm not quite sure why folks are
suggesting that to you. I believe the Berning option is a bit more
affordable at around $6k but you'll have to get on the waiting list which I
think is around 6-8 months is it not? They almost never come up used.

Somehow from your post I didn't get that you were looking for that
particular level of investment in SE options, hence the enthusiastic
second for Wright Sound's offerings. I do love the sound of SET amps in
particular so would also endorse them for what they do have to offer
that is distinctive from other sources of amplification. In very broad
terms this is a greater sense of presence and holography that is quite
hair-raising at best.

Good luck with your quest!

Marco

Pepler,
I apologize for the incomplete response, as Jax2 raised the most important concern: ensuring a synergy between the amps/pre & your speakers. In my relatively limited experience, even a change in interconnects/speaker cables can sharply alter your opinion about the pairing of the two.
One other recommendation: the Audion Sterling MK II. You can find this integrated (actually has a volume attenuator knob, is all) in an EL34-driven version (around $2100), or a 300B version (around $3800). I was very impressed with the airy and expansive sound of these amps. The MK II versions are a big improvement over the MK I. Certainly worth a listen, and sounded surprisingly good on the four different sets of speakers I heard them paired with.
Good luck. Once you go SET...