Need help picking subwoofer cables interconnects

Wow, thanks for all that info Al! That's a good point you bring up about impedance (something I know nothing about). I'm swapping out the S300 with a Belles 150A v2 Reference, which is 100k output input impedance, but that would still result in a total or just over 9k -- still too low compared to the recommended 20k+ load of the Symphonies.

Then again, I've been using the pre with my 10k S300 with great results. Or maybe I'm missing something and I just don't know it.

I would like to use the RCAs since I bought the ACI 60hz high-pass filters to ease the low frequency load on the amp, which is something I'd like to do since my new amp would generally considered a little underpowered for my notoriously demanding Usher BE-718s at 125 WPC.

I will just try both methods and see what works best!

The subs have 2-prong ungrounded plugs.

Question: can I run speaker level inputs from the speaker's binding posts to the subs? I'm running a shotgun pair of speaker cables from each binding post on the amp to the speakers and would rather not add a third cable to each binding post.

"... which is 100k output input impedance..."

errr, I mean "100k input impedance" but I'm sure you figured that out.

I'm swapping out the S300 with a Belles 150A v2 Reference, which is 100k input impedance, but that would still result in a total of just over 9k -- still too low compared to the recommended 20k+ load of the Symphonies. Then again, I've been using the pre with my 10k S300 with great results. Or maybe I'm missing something and I just don't know it.

Well, 9.1K is obviously a lot better than 5K. The main symptoms of the low load impedance, though, are likely to arise at deep bass frequencies, due to the considerable rise that most likely occurs in the preamp's output impedance at those frequencies as a result of its output coupling capacitor. Whatever amount of deep bass rolloff occurs will be more significant when the subs are in place, because the Usher's can't reproduce the bottom octave or so anyway.

It's hard to predict whether or not you will be able to compensate for that rolloff in an acceptable (if not ideal) manner by using the level control on the sub, because the amount of bass attenuation resulting from the impedance rise will increase as frequency decreases towards 20Hz, while the level control presumably affects all frequencies that are within the sub's bandwidth equally.

The subs have 2-prong ungrounded plugs.

Good! That takes care of that issue, as would the fact that the Belles amp does not have balanced outputs when used in stereo mode.

Can I run speaker level inputs from the speaker's binding posts to the subs? I'm running a shotgun pair of speaker cables from each binding post on the amp to the speakers and would rather not add a third cable to each binding post.

I would think that should be ok (with the ACI high pass filters not in the system, of course). There might be a miniscule increase in the amount of back emf from the woofers of the main speakers that would find its way into the subs, but I would not expect that to be significant in degree. I can't envision any other technically explainable and predictable effects that might be significant.

My previous comments about ordinary narrow gauge wire being acceptable for the sub connection would still be applicable.

I would like to use the RCAs since I bought the ACI 60hz high-pass filters to ease the low frequency load on the amp, which is something I'd like to do since my new amp would generally considered a little underpowered for my notoriously demanding Usher BE-718s at 125 WPC.

Good thoughts, assuming the filters are suitably transparent. I have no knowledge of them.

BTW, if you do end up going with preamp-to-sub rca connections, I second the recommendations that have been offered for Blue Jeans cables, in part because they have very low capacitance. The upper treble content of the signals received by the main amp will be affected by the combined capacitance of the cables from preamp to sub and preamp to amp. If the total cable capacitance is excessive in relation to preamp output impedance, upper treble frequencies would be attenuated a little. Although that is usually only a significant concern if cable lengths are particularly long, which I assume is not the case since you indicated in your system description that your room is small.

I will just try both methods and see what works best!

Sounds like a plan! Good luck.

Regards,
-- Al