Update - A buddy of mine had a Harman/Kardon Signature 1.5 amplifier laying around gathering dust. Apparently this is an amp from before HK got out of the 'high end' market. Spec is 2 channel with 200wpc into 8ohms, frequency response 1Hz to 170kHz +/- 3dB. I just hooked it up tonight to run my m60's and am going through a few CD's (Rush - Moving Pictures, Prince - Musicology so far). As I expected the difference is not 'night and day', but there seems to be a big improvement in the mid bass punch, and maybe a little more definition in the soundstage (Stop me if I start to sound like a 'Secrets' reviewer!). One definite difference is the clarity at high volume. I always got the impression that my Denon 3803 was not quite as clean when I pushed up the volume, that feeling has gone now I am using a seperate amp.
So for anyone thinking about going down the same path I would definately say that more power headroom is a good upgrade. Now to find some money to add those rear speakers as well - damned upgradeitus!
