Possible impedance matching issue with subwoofer?

I would add to the key facts that you listed for your gear that the output coupling capacitor in your preamp is 3.3 uf. Many tube preamps use somewhat smaller coupling capacitors, which would increase the likelihood of deep bass rolloff. Many tube preamps also have much lower output impedance at mid and high frequencies, which would further increase deep bass rolloff when used with a smaller coupling capacitor, because it would result in more of a DIFFERENCE between the low frequency and mid/hi frequency output impedances.

Per my earlier analysis, my feeling is that you are certainly not experiencing any major deep bass rolloff, and at most any deep bass rolloff would be extremely slight and would only occur on material containing spectral components approaching 20 Hz.

As I indicated earlier, if it were practicable to go to the high-level sub connections probably the most significant difference would be due to putting the sonic effects of the amplifier in the path to the sub. And that difference, while standing a good chance of being significant, is probably unpredictable in terms of being better or worse.

Best regards,
-- Al

That is what I thought and thanks Al.

If you hook the sub up in parallel to the preamp you will go from an output to input impedance ratio of 94 to only 4. I feel that will certainly result in high frequency sparkle loss. You should look into a buffer between the preamp and sub for the line level inputs. The Musical Fidelity X10 v3 has a high input impedance around 400K that will mate well with your amp. I think the ratio would then be about 44:1 and certainly acceptable compared to 4. I am speaking from experience here, not just textbook. I noticed a definate drop in the highs when I hooked up my sub the same way without the buffer. These buffers don't cost that much. Just get one and try it both ways... you will hear a difference.

Koestner, I'm curious as to what preamp you were using. Perhaps it has an output impedance that rises in the upper treble region.

Also, what were the lengths of the cables to both the sub and the power amp, and what was their capacitance per unit length (or alternatively, what was their make and model)? As I mentioned earlier in the thread, the capacitance of the cable from preamp to sub, as well as the capacitance of the cable from preamp to power amp, can roll off the upper treble (as seen by the power amp) if it is high in relation to the preamp's output impedance.

Best regards,
-- Al

Koestner, I have only seen these placed between the preamp and cd or dac. Can it be placed between my preamp and sub amps and work fine?

I assume the second set of preamp outs go into the x10 inputs and then I output to the sub's plate amps right? Is this better or the same as placing the x10 between the cd player and pre? Will my preamps output impedance be lowered by placing the x10 between cd player and preamp? Lowered in the same way as placing it between the pre and subs......