So, bypass the combo's preamp, and bypass its speaker. You want to inject the signal from a separate, good preamp chain directly into the combo's tube power-amp section. Some small combo amps have an effects loop - such as the VC508. You might need to use a coupling capacitor or transformer, but I have managed to simply hardwire a connection successfully.
Torres has some kits for installing an effects loop.
Bypassing the combo's inadequate speaker and cabinet - Ideally, the combo has an External Speaker jack and a way of disconnecting the internal speaker, so you can drive a serious cabinet instead of the combo's speaker. This way, the combo's speaker is disconnected, and you can send the power amp's output into a serious cabinet or a load and cabinet simulation filter.