Powerful Tube Amp for SF Strads?

You are going to get a lot of suggestions, and since this is Audiogon, most of them bad.

Only a tiny handful of tube amps have output transformers and power supplies of the quality necessary to drive speakers that drop below 3 Ohms in the bass, and they are all extremely expensive. If I were running speakers that present a tough load and just had to do a tube amp, I would look only at the CAT amplifiers, which have the best output transformers I am aware of. They are a real hotrod design, however, and require soldering when tubes fail due to resistors frying upon tube meltdown (which happens frequently because it is a hotrod design). Other options would be the Air Tight Reference and VAC's Renaissance 140's. My guess is that the VTL Siegfried would also work. Generally, you have to be most careful in choosing a high-powered tube amp, as almost all of the well known ones are not as capable of driving low impedance speakers as their ratings would suggest (I also find that high powered tube amps as a rule do not sound very good).

All of that said, I would forgo a tube amp entirely with that speaker in favor of a darTZeel, which will give you better overall sound barring some bizarre synergy issue with the rest of your equipment (I have no reason to think that would occur).

To expand a bit upon the above posts and my own post regarding the CAT amps and their transformers and power supplies, the original CAT JL-1 100 watt/channel monoblocks are gnarling beasts. They contain outrageous hand-wound transformers that weigh 55 lbs. each, the power supplies are something like 1,000 joules per monoblock, and each monoblock weighs 192 lbs. By comparison, the "monster" 500 watt/channel Audio Research 610T monoblocks weigh 170 lbs. each (as transformers and power supplies are both heavy and expensive, the quality of a tube amp can largely be gauged by its weight and cost). I almost stripped a gear helping my friend carry his JL-1 Limited Edition's from one side of the room to another.

Speaking of the JL-1 Limited Edition, it was point-to-point wired and had unbelievable parts quality - it retailed for $50,000 and Ken Stevens only made about a dozen of them (not to be confused with later "Signature" versions of the JL-2 and JL-3 that use circuit boards and cost $20k less). It has incredible finesse and resolution, but at the same time, sounds subjectively more powerful that a 500 watt/channel Krell. I am not aware of a more potent tube amp or a better sounding high powered tube amp. All of that said, like a Shelby Cobra, it was high maintenance and frequently blew output tubes, which took out resistors that had to be soldered out and new ones soldered in - my friend nicknamed them the "Popcorn Poppers" and would not leave them unattended.

The Audio Valve amps are indeed superb and I should have included them in my discussion (I usually do, but forgot). The 400, I believe, is one of the rare high-powered tube amps that truly sounds good. They will be expensive with the current exchange rate (assuming you are in the U.S.).