6L6 power tubes can be a little too clean for the tastes of many. One option is to use 5881's. These are direct replacements for the 6L6 which require nothing more than a rebias after installation. They give a little less headroom, break up a little sooner, and add a little more grind to the amp.
Another option is to swap your 6L6's with EL34's. EL34's have a darker tone, the break up a lot earlier than the 6L6, and their overdriven sound is radically different than a 6L6. If you're looking for a big change in tone, then this may be just what you're looking for. But EL34's ARE NOT direct replacements for the 6L6. You'll need to make a slight modification to your amp so it can use them.
1. Replace the 470 ohm resistor that's between pins 4 and 6 of each output tube socket with a 1000 ohm, 2 watt resistor.
2. Disconnect pin 1 only of the 1500ohm resistor that's between pins 1 and 5 of each output tube socket.
3. Disconnect the wires on pin 1 of the output tube socket.
4. Reconnect the wires that were on pin 1, to the end of the 1500-ohm resisors that are now standing free on each output tube socket.
5. Connect pin 1 to pin 8 of each output tube socket with a solid wire.