Bifwynne, I would certainly give the ZEROs a try and see how that works out.
If you are using the 8 ohm tap, at the 70-200Hz region the power tubes are loaded at half of what is expected for them. IOW, the output transformer live up to its name- it *transforms* impedance, so if it were to normally load the power tubes at say, 3000 ohms if there were an 8 ohm load on the 8 ohm tap, it would be only 1500 ohms if 4 ohms is on that same tap. The power tubes will thus have higher distortion and lower power as a result.
Now with a lot of output transformers, the 4 ohm tap is not nearly as efficient as the higher impedance taps are. So its no surprise that things go downhill using the 4 ohm tap.
If the speaker can be bi-wired, you could use the ZERO on just the low impedance part of the speaker and run direct on the rest of it.
If you are using the 8 ohm tap, at the 70-200Hz region the power tubes are loaded at half of what is expected for them. IOW, the output transformer live up to its name- it *transforms* impedance, so if it were to normally load the power tubes at say, 3000 ohms if there were an 8 ohm load on the 8 ohm tap, it would be only 1500 ohms if 4 ohms is on that same tap. The power tubes will thus have higher distortion and lower power as a result.
Now with a lot of output transformers, the 4 ohm tap is not nearly as efficient as the higher impedance taps are. So its no surprise that things go downhill using the 4 ohm tap.
If the speaker can be bi-wired, you could use the ZERO on just the low impedance part of the speaker and run direct on the rest of it.