Hey titser_marco, I was in the same boat as you are a few months ago except my choice was to keep or sell my JCM900 4101 (100 watt combo) for a new amp. Eventually after lots of research around the net I sold my JCM900 to a friend and purchased a...........Peavey Classic 30!
Ok both are generally good amps. Ive had my JCM for 6 years and brought it frequently to gigs and practice. Lots of bumps, people borrowing it, cranking it, sitting on it, etc. and not even once did it break down. So reliability wise I'd say this amp goes to 11. Can't really comment on reliability of the Peavey since I only had it for a month.
Tonewise, the JCM900 had excellent distortion. Thick and warm, not super high gain but enough to cover your basic punk and hardcore distortion. Listen to Silverchair's Frogstomp, Green Day's Dookie, Thursday's Full Collapse, thats a JCM900 speaking. But take note, to get that sweet drive the amp has to cranked or at least on 5, at bedroom levels they sound ok. Yes these amps use diode-clipping and not a straight tube path to help it distort but that did not bother me since its basically the same principle as adding a dirt box on the clean to get your distortion. Besides the Marshall Jubilee, Slash's amp, also uses solid state devices to add more gain. This is why cranking this amp sounds better since you start to give the power tubes a good workout. If you want pure tube tone out of this amp you need to crank the clean channel. I believe diode clipping is not used for this channel so your just hearing the tubes. If you decide to get this amp, look for ones with EL34 tubes. Mine came stock with 5881 and I had it modified by Lito Bote to fit EL34's. Many would say that these tubes are the heart of the classic Marshall sound. It made the amp more balanced sounding.
The reason why I sold this amp was because I thought the clean was sterile sounding. No matter what I did, it lacked depth. So I needed an amp with a good clean channel and drive and ended up the the Classic 30. The clean, although not as sparkly as Fender, had a lot of body and depth. Very warm and the highs are sweet, not piercing. It takes pedals very well, I'd say better than the JCM. The drive channel is where its also happening. It does not have the gain of the 900 but its voicing IMO is similar, again thick and ballsy however when cranked, the highs can get too sharp. To combat this I thought about a tube upgrade so I contacted Bob at eurotubes.com and ordered a set of tubes. He only stocks JJ tubes and based on other forums on the net this brand is supposed to be the best for the Classic 30. And yes you can order them from the Pinas. A set costs $62+$15 Global Priority shipping + P1200 customs and taxes. Took about one week and a half to arrive. Slapped them on the amp and the stabbing highs on the drive channel was gone. Overall the amp sounded great after a tube upgrade. And what I know is that the Classice 30 uses an all tube path throughout so you're getting pure tube tone from both channels. If you think it sounds good stock it sounds great after the upgrade. Again IMO.
So it all boils down to what you're after. If you play mostly rock/punk/hardcore with a lot of distortion and little clean I'd say get the JCM900. If you want a good all around amp that works well on both channels and can take pedals well go for the Classic 30 w/tube upgrade. Don't let the 30 watts fool you, this thing is LOUD! The clean breaks up at around 5 but thats already too loud for most band settngs. The volume on 3 on the clean can compete with a heavy drummer. Good luck and most of all take your time. I never regretted selling the JCM for the Classic 30.
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