I powered the PSB 2B (forerunner to the B25's) with a vintage NAD 3020 with 20 watts. It did beautiful job. If fact, if I wasn't so lazy and needed things like a remote, I'd still have that set-up.
If you liked the PSB B25's, I've read that the Monitor Audio 2B's are very similar - possibly a bit more detailed and, I believe they are 8 ohm.
John