PaoloJM

To me this sounds more like a problem in the heater circuit.
All symptoms point there: valve not lighting, taking a long time to heat, low gain, very little overdrive.
DSLs are prone to poor solder joints making intermittant connections. Pins 4,5 and 9 are the heater pins for the preamp valves. I would start by touching these up.
The first two preamp valves are feed from a DC supply. Perhaps there's a problem there? Faulty rectifiers (BR1), filter caps (C1 and C2) again check for dodgy solder joints?

Impedance mis-match wont affect the preamp stages.

If you don't have any experience in electronics then take your amp to a technician to have this done. The DSL is a particularily complicated, poorly made/designed amp and not the amp a beginner should be working on.
It also contains dangerous voltages, and you will be the amp live and open for testing.

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