It is not designed for a standard rack mount space but for 500 bucks take a look at the Decware Zen amp; it has the right power for your 99db speakers.
Aball is right. 99dB sensitivity is quite high, especially when your sub is powered, so you don't need to look at 25-watters. 3-10 watts will simply blow you away ... for all of the right reasons. Just do some crit research. I wouldn't spend mega-dollars at first, and I don't think I'd even consider 300Bs at this point, at least not before you know that tubes are where you intend to stay.
It occurred to me that Ty may be building you a 4 ohm loudspeaker. If the dual 10" midwoofers are run in parallel, then it will likely be a 4 ohm (or below) load. In that case, 99 dB efficient at 4 ohms is equivalent to 96 dB efficient at 8 ohms. An 8 wpc 300B SET amp may not be enough in this situation.
For something cheaper than $500, you might consider a Dynaco ST70 (lots of them on the market, lots of tweaks/mods).