As Tim mentioned, there doesn't seem to be too much of a need for a specific amp for a low B. I play a 5-string through a Peavey TKO 115 Combo that I bought 12 years ago and it does the trick (and sounds better than the new version).
The real thing to notice here is that bass frequencies require more wattage to properly project. This is why a 100 watt guitar amp is ridiculously loud, while a 100 watt bass amp would be left in the dust. As such, adding lower strings will require more power as well. Most bass amps are capable of handling a 5 or 6. I haven't really tried out a 7-string, so I can't say for sure, but it should work just as well. If you need to get really loud, you may need a little more wattage to support those frequencies. One possible thing to try would be to use a sub-woofer to use in conjunction with a set of 10's. Set the crossover to a good spot and you will get super solid low end with great clarity in the mid to upper range.
So a higher wattage will give you more power to support the low frequencies. You would essentially be looking for higher wattage and to make sure that the frequency range of the amp and speakers specify that super low frequencies are included in the specs. Also, a low F# like that gets close to the lower limit of human hearing, so you're almost looking more to feel the notes than hear them.