i had a Marshall 20 watt valvestate ...
i ddnt shop and try on that many practice amps, but what can i say? that amp kept me from GAS for more than 5 years. it never farts even at full volume, and you dont hear any cracking like those japanese surplus amps. at room volume, the valvestate can make you feel as if you emulated the sound coming from pantera, slayer and metallica records - of course it is not close... but still, if you dont know any better, you'd think you are close (then you go to onstage and realize you dont have the power to match the un-miked drummer).
the only reason why i sold it was because i got me a 100 watt b52... its similar to the Marshall Valvestate, only louder and you can really feel those bass notes pumping out of your speakers (something practice amps have a difficulty doing). i believe the b52 is a very good marshall clone...