ST-70 or MkIII.
Agree with Tvad, tubes will have trouble driving those speakers becuase of the impedance dip and swings and maintaining balance from highs to lows. That B&K is a pretty good amp for driving those speakers. I love tubes and only use them in my system, but not all speakers will mate well, yours are not likely to be a match made in heaven with most tubes amps, there are exceptions (Music Reference RM9SE) but they will be expensive. At your budget, I would stay away from tubes, unless you change your speakers first.
I agree with Tvad and Pubul57 that your current speakers preclude most if not all tube amps in your price range.
One possibility is to go to a Class A solid state amp. It won't sound the same as tubes, but may give you some of what you're looking for. I had good success with the Monarchy SM-70 Pro, using two in a monoblock configuration. They'll handle the 3 ohm dip just fine. They can sometimes be found within your budget for a decent pair. Make sure they're of similar vintage so that the input sensitivities match.
Any of the better but used ultra linear or push pull intgrateds from China should not exceed your budget. The models biased for EL 34s are a good place to look. Several of these have established themselves as premium amongst the great rabble. Email me I can give you the low down,
If insist on new you are stuck with few unknowns many on ebay for instance have unusable low output power. These amps running less than 20 wpc in PP would probably fail tp deliver clean and abundant bass. This could turn you away from tubes altogether coming from a Byston .
I would wait until I had an opportunity t get a rather more costly amp with overbuilt power supplies and the ability to deliver high current regardless of the wpc promised.
A word of advice about figuring out power is that you can't really do what I just did which is to slap an arbitrary figure on it like wpc. There are many examples of low numbers for wattage. That easily outmuscle bigger number amps due mostly to current capacity.
The ST 70 is a very good suggestion.
But if you realize at the start that these amps are more about imergy and detail good definition and a good reproduction of middle frequencies I would look for Mini Max, Ming Da, Prima Luna, and some of the old players in this arena such as Antique Sound Labs, they can come up with a real gem sometimes but it's irratic or even Jolida but I never liked them all that much.
Except for some fine tuning of the user interfaces like the Alps velvety feeling volume pot on a Prima Luna or the Aesthetic and bigger trannies on a Ming Da. The amps are remarkably blase and only the luck of the draw seems to get you something that's a jackpot piece. I own 4 Opera Consonance amps two are good one very average that is not bad but one is spectecular. Cyber 800 monoblocks are world beaters in my view. No they aren't quite the same as my less wpc Jadis integrated Da 60 but very very good close.
Every poster buy rulenot onl;y should but by rule of law must tell you trust your self. No one else except a very sig interested sig other should dictate your choice. If you don't the significance of pissing her off will be made very clear.Otherwise the best is what you like best.
Go out and try as many as you can. Go to a show etc.
My one very strong recommendation is to avoid the tube pre and SS amp folly. seriously don't believe the clatter ignore them and do not go with tubed preamp section running a transistorized power amp. if you really mean you want to hear tubes doing what tubes do best.
I tried variation after variation of that popular theme cltching my teenage fantasy of a big block V8 400watt per channel monoblocks and never managed, not once, to get the incedibly good sound tubes can give you.
For realizing the tube sonic the path is clear. You must use a tube power amp or tube power output section. Ignore the nattering naebobs to follow claiming that a tube preamp is all you need . It is absolutely false. At most it may get you about 10 or 20% of what you would discover if tube power is used. Let me say as a committed tubaholic except for all the time with a very few specific applications and speaklers where SS is what is what I want.
You may think you are hearing the "magic of tubes" but when you actually put together a great synergistic tube system the tube "magic" is not ver easy too hear. It will hit you like a ton of bricks nothing subtle I promise.
I was taught this by years of experience of half way frustration and puzzelment even with a tubed integrated of moderate quality nut tube power there was something missing. The amp was mediocre just not right.
Then an audio buddy who is very sharp and knew the answer.Simply announced one day that all I needed were bigger tube power amps.He knew my preferences well and the SET tube experience was not the goal. He was so right that I wonder why he waited to tell me,
Oh Well.. I am telling you right off the bat.
Clearly a tube pre with tube power is even better at getting the elusive tube sonic.
However the demands of the speaker and music on the amp will define limitations of an under powered tube amp with insesitive or just impedance mismatched speakers. There are too many small to medium tube power amps, that just might not have enough jiuce, however you measure it, to play what you like, at a volume you like.I have a trail of literally a dozen amps to prove it.
I must admit since finding the solution I bought two set ups that are Nirvana, I go back to listening to them after a day off and still marvel. So much so that I never bothered to fiddle around with T, ICE amps, or other switching amps my first impressions of them were that they were brilliant clean and neutral but I wanted emotion images etc. Those latest amps are not for me.Not even close. There are so many who have found them to be their preferred sonic that I can't drive you to or away drom them. I just lack of experience. But you did say tubes.
A final word of caution, tubes can be a very slippery slope in terms of using various tubes each brand and compatible type lends it's own sonic signature and I got into collecting them. The variety is endless and tubes that cost 20-50 bucks a pair are now several hundred depending on which ones of course. You may do well to change them once or twice and settle on not knowing every tube out there.
Anyone want to buy some tubes? No I am kidding A gon forbids selling on foums. I merely wanted to point out that I ended up drowning in them.So be careful
Based on the Stereophile measurments of the original test, I'm kind of surprised you get good results with the matchup to B&W.
They had some... difficulty... at 4 ohms, not to mention lower.
Also, Please keep in mind that the real measure of a loudspeaker's 'goodness' of load is a combination of impedance AND Phase Angle....the amount the voltage and current demands are 'out of sync' . This number is usually measured in degrees and can be expressed as a number called 'power factor'.
Large phase angle combined with low impedance is a deal breaker for many amps. I don't have the phase data for the B&Ws but if it follows there 'norm' it can get to 45->60 degrees. This is not a trivial number.
And, as a final note, Tube amps do not like certain reactive loads. I don't remember off-hand if it is Capacitive or Inductive which they don't like, but this too, makes a difference. It may make a difference if the amp is either direct coupled or running thru an autoformer, I have no information on that.
A version of a tube amp that will work is a nice Hybrid tube amp. Tube first stage and SS second stage. Monarchy makes some nice ones 100 and 160. They do show up if you keep watching.
Blue Circle also makes some nice ones that can be had used for close to $1000. None right now on the Gon, but I have seen their 100 watt hybrid time and again used. BC24 and I think the 22 or 28? Forget.
I have owned both and they are great.
Electrocompaniet makes great SS amps with very musical, tube like presentation. AW220 - Very warm yet detailed and so musical. These do show up on the Gon!
A Moscode 300 or 600 or Counterpoint SA12, SA20, or SA220 would fit the bill in a hybrid design and still stay within your price point.
But I have to agree with Rleff, B&K's higher powered amps are extremely tube like sounding. Many years ago, I had a B&K EX442 Sonata and it was almost impossible to tell the difference between that an a pr of Paoli M60 monoblocks (8417 highly moded Dynaco mkIII's)