I have a single 12 extension cab for my DSL40c. I've even used my cabinet. And it surely brings the amp to life a bit....but for me, it kind of defeats the purpose of having a small, manageable combo amp.
I think it's a good amp overall, but it has its limitations for sure, most of which you discuss in detail. . I've replaced the tubes, but I've never gotten it back to where it was when I first brought it home.
At the band room, I use the "house" ibanez tube amp. No idea of the model, it sucks, and I hate it. But, that little ibanez amp is the number one upgrade to making my DSL40 sound great!!
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What tone are you looking for??? From the DSL40C...