You might consider bi-amping them with a SS amp for the bass, and small tube amps for the ESL panels. Matching the output volumes might be a little tricky, but with enough fiddling, you could probably get it to work...
Go for used tubes. McIntosh, maybe some old Quads, Manley, Jolida. I doubt you're going to find any decent used tube amps (even small ones) for under $500 though. Tubes, even small, low-power ones, are a pricey proposition.
And you've got to consider the long-term cost of running tubes. With tube amps, it's not the initial cost that kills you, it's the cost of replacing and maintianing the tubes over the life of the amp that drives the nails into your check book. A matched set of 8 KT88s or 6550's can cost almost as much as a pair of Vistas...
Of course, if you love the tube sound, but don't mind "cheating", you might look into some old Carver or new Sunfire amps. Low cost, high power, and tubey as all get out, with the added bonus of being 100% solid state.
Or you might consider just using SS gear all through the chain, and getting a tube buffer to stick between the preamp and amplifier. That can make a LOT of difference in some systems, especially with very fast, analytical amps like Krells or Levinsons.
--Richard C.