I had a Debut Carbon DC for a long time. It might even still be in my signature. I only sold it because a good friend was lookign for a quality deck and I have an emotional attachment to my Dual 504, and my 3 other record players are in the kilo-buck range given the upgrades I've done. I was always pleased with the Debut Carbon DC. It wasn't on the level of my heavily modified thorens or technics SL-1200 mkII, but I could buy 2 or 3 of the pro-ject for what it would cost to get a similarly equipped thorens or 1200. I felt the Pro-ject was superior to the similarly priced rega, and superior to the fluance and u-turn offerings in the closest similar price range. I don't know why there seems ot be such variability in experience on the pro-ject tables. Mine has been nothing but positive.
The OP is wondering about SQ on the table he has with some of his punk and metal records. My thoughts (which will get you a cup of coffee if you also have $3.50):
1. the type of music you are listening to is notoriously poorly recorded in many cases. Even some of the bigger names (both in terms of band and record label) have really bad SQ on their master recordings. Some of that is because the lo-fi sound is a badge of honor to some in the metal/punk community. Some of it is access to quality mics and recording equipment, some of it is access to quality sound engineers in the studio. Keep this in mind.
2. While I had my Project turntable, the biggest upgrades I did were to (A) put it on a wall shelf, (B) get a really nice phono preamp and; (C) going from red to blue on the ortofon.
2(A) The wall shelf improvement in SQ was way more than just eliminating foot-falls and resonant vibrations from whomever might be walking around. It added clarity and cleaned up the bass, which allowed the lower midrange to come through with more detail.
2(B) The upgrade to the blue stylus was another level of detail added. On some records it was almost too much, but those are records that I know are already "hot" in the higher frequencies.
2(C) I won't delve into the bottomless pit of phono preamp discussion, but suffice to say if you aren't using something on the level of a schiit mani or something on that level, you are going to miss out. A good phono preamp is also nice because you can take it with you when you upgrade turntables.
3. There is a bit of debate about "how to listen" with respect to the dust cover being up or down. In my experience, the exact thing you are describing (muddy mid range, good performance when "not busy" music is playing with a steep drop off when more congested passages are coming through) is characteristic of having the dust cover down while listening. I ALWAYS have the dust cover UP while listening now. That goes for ALL of my turntables. I also moved the living room turntable out from the "cubby" shelf into which it fit so nicely. It seems that when the immediate vicinity around the turntable is more closed in, the sound ends up getting a little muddy.
4. If your signal cables are hanging across other cables, especially power cords, that will give you a bit of haze in the sound, especially around 60Hz (an harmonics thereof) which can translate to reduced definition in the midrange (from my experience)
before ditching the pro-ject, I would try mounting it on a wall, playing with the dust cover up/open, and make sure you have the signal cord reasonably away form other wires. These are all "free" upgrades, if you can do a little DIY. Then I'd also look at getting a better phono stage. Decent ones can be had for under $150.
Good luck!