By adding a power amp to my system, will I get more detail at a lower volume/power levels?
You will get more detail at lower power level's if you get a better amp and speakers. The amount of power you need depends on your speakers.
Your B&W 685s are rated @ 25 to 100 rms so your B-60 amp should be all right
the speakers only open up at about 50% on the volume/power knob, at which point the bass can start to get a bit muddy quickly
60 watts only goes so far, but should be good for your speakers.
At this point I don't know if its your amp or speakers. Could be because your speakers require more power, most of the power goes to the bass, that's why you hear that distorting first but pushing your amp makes a lot of things sound bad.
So a more powerful amp could be what you need. My advice is to get a high Quality lower cost power amp first. This should help a lot, then down the road get better speakers.
Here on Agon I found
musical fidelity a3cr reduced!
I personally own a a3cr driving my 6 ohm Electrostatic speakers. The Amp is in total control of these hard to drive speakers. With the a3cr I hear a lot more low level detail than I heard with my other amp, that's why the a3cr received a class A rating from stereophile.
You can look at all the amps listed for sale, the a3cr to me is the best value. You have to spend quite a bit more to do better.
Also get a good power cord and a good interconnect to hook up your new power amp, money well spent.