Not only are your speakers vicious (and tube-unfriendly) loads, as George points out above, but in the bridged mode you are using your amps will "see" speaker impedances equal to half of the already very low values he cited. Which can't be good for the sonics of the amps, even though I note that you indicated in
this 2015 thread that Dan Wright had "optimized these amps for bridged mode," whatever that may mean.
Also, as others have alluded to there are a great many ways in which using solid state and tube amps in a biamp configuration can do more sonic harm than good. And even more so when the speaker is inherently not tube friendly in terms of either its impedance characteristics or its power requirements.
My suggestion is that you consider trying your existing amps in a passive vertical biamp configuration, with each amp operating in stereo mode rather than bridged mode. Your speakers have low impedances in much of the bass region, and the 250 watt/4 ohm rating of your amp in stereo mode could very conceivably be adequate, especially given that each channel of the amp would only be supplying current and power in part of the frequency range when used in that configuration.
The one caveat that occurs to me related to that configuration is that I'm not sure about impedance compatibility with your line stage. In that configuration the line stage's output for each channel would be driving the inputs of two amplifier channels, and would therefore see a load impedance equal to half of the input impedance of each amp channel. The input impedance of the amp is spec'd at "15K at 50Hz; 23K at 1Khz." The LS36.5 has a specified output impedance of 110 ohms, which would seem compatible with a load impedance of 15/2 = 7.5K, but as with many tube preamps the 110 ohm figure might be considerably higher
at deep bass frequencies. That would be a question for Dan.
Also, assuming you are using balanced interconnections between the line stage and the amps you would need an XLR splitter, such as
these.
In your older thread that I referenced earlier, BTW, I had advocated against a vertical biamp solution, but that was under completely different circumstances prior to your purchase of the Modwright amps.
Good luck. Regards,
-- Al
