Possible impedance matching issue with subwoofer?

Not sure what kind of sub you have, but with the ones that have both line level and speaker level inputs, the sub just 'samples' the input signal (yes, the RCA inputs too). So your preamp isn't really 'driving' the sub's amp(s) per se; so impedance mismatching that might possibly normally occur just isn't a factor. If you're still concerned, contact the sub manufacturer and find out for sure.

No Al, that's not the way it works (anymore ;--) Yes, in the old days, 'plate amps' were basically full range ss amps that were put on a 'plate' chassis so they could be mounted on the back of a speaker -- quite often a full range monito speaker (not a sub) which (self-powered) was more convenient in studio environments (no long speaker cables; using balanced signal lines of course.) And BTW, do you remember when many subwoofer (drivers) were made with dual voice coils, to keep the preamp's L/R outputs separated?

But today's powered subs (all the ones I know about, anyway,) including the subwoofer section of certain hybrid speakers (like the MartinLogan models that have a powered woofer section) use Class D ss amps which are fine at low frequencies, but aren't meant to be driven with a full range signal. These modern subs often include an LFE (Low Frequency Effects) input which DOES drive the plate amp directly, from a single (mono) interconnect, since it's assumed that the LFE signal comes 'pre-configured' from a surround processor and only contains mono low frequency information, already with the desired slope and cutoff determined by the processor.

The L/R speaker, and L/R line level inputs on these subs are "connected" to the sub's (Class D) amp through some kind of a buffer which also a.) combines the L/R signals (while keeping the preamp's L and R signals separate) and b.) limits the frequency range going to the subs internal amp. That's why the slope, phase, and often even the level controsl on the sub do not affect the LFE input signal, which it's assumed are configured at the processor. My point is that the "input impedance" at the buffered L/R line inputs (and L/R speaker inputs) of most subs today is 'set' by the sub's designer to accept almost any preamp's L/R main output without doing "damage" to the music signal due to accidental impedance mismatch.
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Al, your erudite response notwithstanding, it's unlikely ANY manufacturer these days would leave it to the user to make sure his product performed optimally ;--) I don't think Mr. Grannyring should be concerned in the slightest.

As he already said,

The system sounds very good as is, just wondering if perhaps I am onto something I have missed with possible bass performance improvement.

and so my response is, don't go looking for problems where none exist ;--)
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