Subwoofer for classical music listener

The reason I'm asking about connecting cables is that I'd like to smooth out the highs which are a little bright.

Yup, cables can make difference.
I forgot what you are using for your speakers and don't want to hunt through this long thread.
Also, what lengths are you looking for? 25' will definitely put a damper on sound quality-especially if you are using inexpensive cables.
Almarg gave me a good recommendation for sub cables.

Bob

"Where would I attach the power cable?"

rv- I'm just referring to swapping the supplied stock cable your sub came with?
The cable game is contentious, so I've learned over the years to tread lightly.

Personally, I've joined the camp-power cords first. The "highs" you're referring to, 
Im assuming you've exhausted all tweaking of the subs adjustments?

RV--Are you running 25' rca cables from your preamp to your sub and then 25' back to the amps?  Do you need them that long?  If not, shorter, the better.  I have a couple short SVS "SoundPath" cables.  They seem to be well made for the money, and, to my ears, do little to nothing to the sound and don't pick up noise, as they should and as almost all decent cables do, imo.

For the more technical folks who may opine on the length and potential impact--here are the SVS specs on the "SoundPath" rcas:

Conductor DCR = 77 mΩ/meter
Shield DCR: 30 mΩ/meter
Nominal Impedance: 110Ω. Cable and RCA connectors are impedance-matched to minimize Voltage Standing Wave Ratio (VSWR) and Return Loss.
Nominal Capacitance: 39 pF/meter @ 1kHz
Rated Bandwidth: DC-5.0 MHz (3M cable) with ≤1.09 VSWR and ≥27dB Return Loss

RV, are the 25 foot cables connecting the preamp to the sub AND the sub to the power amps, or are they just being used for one of those two interconnections?  And if the latter, which one?  And how short would it be feasible for the cables to be for each of those two connections?

Best regards,
-- Al