Adding Tone Controls?

Rlwainwright has suggested your best approach.

Tone controls are band aids that have no place in a well thought out and designed system.

Shakey

To each his own. I am happy to have them. Its the software that is the problem, not the system. At least that's my take on reality.

"My system sounds wonderful when playing well recorded jazz, classical, or "audiophile approved" material. Unfortunately, mass market pop frequently sounds horrible, with screechy splashy highs. It's obviously recorded with a built in bias to be played on car radios or lo-fi mp3s."

Welcome to the club. Your situation is very common; its one of the negative aspects of having high resolution audio equipment. There are a lot of different things you can do to fix the problem. I believe the best, and most effective fix, is to get an EQ.

"Oh for heaven's sake, he's already listening to abysmally recorded albums and you're gonna worry about the supposedly deleterious effects of an equalizer? Some times I wonder if some of you guys made it thru grade school."

I couldn't agree more. I have an EQ that I use for the same reason. You may want to look at a Behringer Ultracurve PRO DEQ2496. I have one and get great results with it.

Behringer DEQ2496 is exactly waht you want.
It is a digital device which can be placed between your transport and DAC.
If (like most) you have multiple outputs on the transport, you can send one to the DEQ2496, and another to the DAC directly. (the DEQ2496 'likes a toslink or AES-EBU in, and i find the AES-EBU out to be the best.
Then you can just flip a switch on the DAC to have the DEQ2496 in or out of the path.
It is a very complex digital device which can do all sorts of tricks for you. including using as an EQ.
Check it out.
I have one I bought a few years ago for $200.
They are now more expensive, but $315 (on Amazon) should find you one.
Steep learning curve, so you HAVE to use the manual!! but worth it.
The BEST low cost digital EQ around. And like i mentioned, it does a lot more too.