suitable ob drivers for low powered tubes

Depends on how low the power you would need something of at least 93dB per watt efficiency for 8 watts or 99dB for 2 watts.

Supposedly you need a driver with a high Qts to work well on open baffles.

I'm no expert but I think you could try vintage drivers for your budget. Have read great thing of the Goodmans Axiom 201 on open baffle. These are below your budget and are around 99 dB efficient. I've at one point tried Altec 403As on open baffle with good results and you could try to pick a pair of Altec 755Es for your budget. These are around 93dB efficient and have developed a cult for open baffle users particularly in the Orient.

There's a project by Thorsten Loesch for modifying an Eminence Beta 12 LT. Do a google search for Afterburner. These go for less than $60 at parts express and I believe are 99dB efficient.

Good luck
Thanks, I will look into all suggested. Especially that Eminence mod, I have been thinking of doing a Bastanis clone and the Beta 12cx is the starting point. The active bass seems like a easy way to fill in the problem area for open baffles, and I like the minimal crossover utilized in the Bastanis method.