The outright power of a subwoofer, and any thoughts of being too much, is irrelevant in any system, any room. Whilst power is important for high output levels, extra power is also needed for low notes, and the control needed requires more power from the amplifier. I notice though that the 650watt rating of the Boston is peak, so any continuous output is going to be around 200 or thereabouts.
I'd put forward the Tannoy TS2.10 as an option. As well as a 10" driver, it also has another 10" driver acting as a passive radiator - this will shift more air, resulting in better efficiency and more presence.
Of course, adding the cost of the sub to the cost of your speakers means you can get better quality speakers. Adding a sub to any given speaker doesn't physically improve quality (although there is a perceived improvement in quality), it will only extend bass. Have a look at Dali Menuets, KEF R100, or Monitor Audio Silver 1 or 2. If you're more I tested in smaller speakers, but if you're after a floorstander, try the Dali Zensor 7 or the Monitor Audio Bronze 6.
See if you can arrange an audition to compare the sound you'll get from a pair of floorstanding speakers to floorstanders with a sub, as you may prefer the sound of one approach over the other.