3 possibilities I can think of:
1-change to a more aggressive tube, ie GE vs mullard
2-go to a powered sub/crossover running between the pre and amps. This will give a ton more power and shouldn't be an audible difference crossed below 100hz. According to the owner of Janis (subs, crossovers and sub amps), if you cross over the system at 100hz, you will effectively triple the output power available to your speakers!
3-Get a more detailed cable-I love nordost
1-change to a more aggressive tube, ie GE vs mullard
2-go to a powered sub/crossover running between the pre and amps. This will give a ton more power and shouldn't be an audible difference crossed below 100hz. According to the owner of Janis (subs, crossovers and sub amps), if you cross over the system at 100hz, you will effectively triple the output power available to your speakers!
3-Get a more detailed cable-I love nordost