JCM800's are great amps and I wouldnt go running off to buy something new without trying out a few other things such as experimenting with the tone knob on your guitar and/or switching to neck or middle pickups for clean sound. Also Tele correctly mentioned trying out an EQ pedal - the Boss one is inexpensive and very tweakable (I've used one before with great success). I love Marshall tone, excellent for heavy and clean IMO.
What kind of guitar and pickups are you using - that's where the core tone is created in the first place.