How do your voltages compare with Sluckey's schematic?
Farting bass notes usually equals grid-blocking at the output tubes. Are you using any pedals when this happens (especially boost pedals or distortion/fuzz pedals with high output)? Is it only when you're playing very loud?
Usual cure for grid-blocking is to reduce the size of the coupling caps from phase inverter to output tubes (0.1uF originally in this amp). Yes, that will reduce bass.
The total system responsible for grid-blocking (if that is actually causing the bad sound in your amp) includes the coupling caps, the 220kΩ bias feed resistors and the 1.5kΩ grid stoppers on the 6V6's. Grid-blocking happens when you slam the output tube grids with a very large signal, such that the positive peak of the signal is greater than the bias voltage (or a peak greater than your 34v of bias). The grid signal overcomes the 34v of bias so that the net grid voltage is positive. That causes grid the grid to attract electrons from the cathode, which then flow out through the bias feed resistors and bias supply to ground. But large-ish bias feed resistors tend to slow that drain of grid current, which charges the coupling cap plate closest to the output tube grid negative. The grid is now more-negative than it would be due to the bias alone. When the signal swings negative, it will cut-off faster than it ordinarily would because the net grid voltage is more-negative than it would be due to bias and signal alone. The whole cycle builds up as long as there is a huge input signal and causes some pretty ugly clipping.
Interestingly, output tube grid current also causes distortion in the phase inverter. Normally, the grid looks like an open-circuit so the phase inverter's load is the 220kΩ bias feed resistor. While grid current flows, the output tube grid looks like a much smaller resistance to the phase inverter. The end effect is a momentary reduction of the phase inverter's load, which can clamp the peak output of the inverter for an instant. One part of the phase inverter output wave gets changed, which amounts to distortion (signal out not same shape as signal in).
Once the too-big input signal is removed, it takes some time for the cap charge to drain away, which is determined by the time constant created by the coupling cap and the bias feed resistance. Make either of those parts larger-values, and the time constant increases as well as time to recover from grid blocking. Reducing their value speeds blocking recovery, but making the bias feed resistor smaller may load the phase inverter too much (depends on the particular amp design). So typically the coupling caps are reduced.
But we do have those grid stoppers. Making them bigger slows the electrons coming out of the grid, which is the same as reducing current (definition of current is 'rate of movement of electrons'). You could try inserting bigger & bigger grid stoppers (start with, say, 10kΩ) to nix the grid current, but large-enough values will reduce treble response at the output tubes.
All-in-all, smaller coupling caps or bigger grid stoppers. You could try 100kΩ bias feed resistors, but they may not be enough to fix the problem and I certainly wouldn't go lower than 100kΩ in this amp (lower needs a phase inverter redesign).
