Need help picking subwoofer cables interconnects

Is the equipment list in your system description up to date? If so, there is a larger issue that is present, which makes Foster_9's suggestion of using speaker level connections particularly appropriate.

Preamps that have two sets of outputs usually (although not always) have the two rca jacks for each channel simply wired together just inside the rear panel. This photo appears to confirm that is what is done in your particular preamp.

According to the S300 manual and according to this thread, both the sub inputs and the power amp inputs have input impedances of 10K (10,000 ohms). Therefore if you connect one set of preamp output jacks to the sub and the other to the power amp, the preamp's output stage will see a combined load impedance of 5K (two 10K's in parallel). That is too low to be optimal with respect to the preamp's nominal output impedance of 600 ohms, which probably rises significantly at low frequencies. The result will most likely be bass rolloff and other frequency response irregularities in BOTH the main speakers and the subs. It's not possible to say quantitatively how serious a problem that would be without having an output impedance vs. frequency curve for the preamp, but it figures to be significant.

The sub manual indicates that it is supplied with speaker-level to rca adapters, and the Audiocircle thread I linked to above indicates that the impedance of the adapter is 22K (I suspect that it is just a series resistor). That in combination with the 10K input impedance of the sub will present a completely negligible load to the power amp.

According to the manual the adapters can either be connected at the power amp output terminals, with an rca cable between adapter and sub (connected to the adapter via a female-to-female rca adapter), or at the sub input terminals, with speaker wire running to the amp. Since the amount of current that will be drawn by the 22K + 10K is insignificant, you wouldn't have to use typical heavy gauge speaker wire with that approach. Plain two-conductor 18 or 20 or even 22 gauge wire obtained at Home Depot would most likely do fine, IMO.

Regards,
-- Al

Important follow-up to my previous post:

I just noticed that the S300 appears to be a fully balanced amp, and its manual therefore has strong cautions against connecting any of its speaker output terminals to ground. If the subs have 3-prong power cords, connecting the subs to the S300 outputs would ground their negative output terminals, unless a 3-prong to 2-prong adapter is used on the sub power cords to eliminate the safety ground connection. As you probably realize, that would present a small but non-zero safety risk.

The picture on page 5 of the sub manual appears to show a 2-prong power plug. Hopefully that is the case.

Regards,
-- Al

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