A couple of question...

I just hooked up my Bel Canto S300 amp to my Benchmark DAC using RCA inter-connects. Good one's I think, Eichmann w/ Cardas. What, if anything, would I gain by using a balanced xlr inter-connect?
Also,I use the Wadia 170i as my main source of playback and find this combo to sound a bit crisp, like a Feb. morning on the streets of NYC. Freezing, yes but great just the same.A bit bright. Sharp. Would a tube pre-amp take some of the bite out of this set-up or is it just me?
Traditionally the primary advantage for using balanced (XLR) interconnects is to keep the noise floor low in long cable runs. Microphone cables which operate at low voltages and have long strings of wire from their position by the musicians to the mixing console is a classic use.

Balanced connections also give more output (6 dB as I recall) than unbalanced.

If you have a short run there may not be much to gain. However, remember in this hobby there is no issue too small for some enthusiasts. You can only try and see if it is worth it to you.

Regarding the "crispness" in your sound, I might suggest borrowing a different DAC. The Benchmark does have a reputation for having a well-defined edge. I use a Lavry DA-10 and really enjoy it.

I had a Bel Canto S300 for about a year and thought it was a great amp. I had no problem with the sound but went back to tubes simply because I'm a tube fan from way back.

You didn't state what speakers you are using. It is a bit of fashion in the high-end these days to have a highly defined, sharply etched sound. The Benchmark in combo with the speakers may simply be highlighting this quality.
M1sst1, thank you for the detailed explanations.
My speakers are Sonus Faber Concertos, certainly light in the low-end with a smooth mid-range and well defined highs.
I would imagine a tube pre-amp might lessen the razor sharp sound of the Benchmark.
I had a dac3 hooked into a s300, it did not sound etched or harsh. I also had a benchmark before the dac3 and chose to keep the dac3 for this reason. I also had a cary slp-88 preamp in the chain for both. While playing around with taking the preamp out of the chain, I found the sound to be clearer, smoother, and more detailed with just the dac3 and the s300. Then a few months ago, I got a Jolida 502brc. It bested the s300 in every way possible.
So in adding tubes to your system, you could probably take the edge off with some warm sounding tubes, ( red base 5692 would have done it in the Cary )but it may depend on the quality of the tube component ( I am aware Jolida is not the be all of tube amps, but the slp-88 is entry level for Cary ), and the tubes you use. It could just be the sound of the DAC1