I recently picked up the SRD-7/SB along with a set of SRX Mrk3s.  Strangely, a previous owner had attached a 'phone jack to the SB so I used a spare 'phone amp.  The sound was very weak and spare through the under-powered connection. I got a slightly more powerful T-amp, 12 wpc, and the sound is much improved though still lacking in some ways.  Eventually I'll hook up to the tube amp and see if there is a similar improvement.