Can't listen right now, as my setup is currently disconnected so I can put my monitors on new stands tomorrow, but this is really the only forum I've ever heard the 6505+/5150 II described as being "tighter" than the 5150/6505. I've owned over 15 of the amps in the 5150/6505 series (obviously numerous repeats of the same model) and the 6505+ was always looser than the original, as well as smoother and less aggressive. This is what gives them their "bigger" sound. The 5150 I have now is one of the tightest sounding amps I've ever heard, and I've owned almost everything.