Why are the vocals on some records hidden behind the music on my system?

I also suggest a different cartridge; I should think a LOMC would be the way to go, but that's my preference.  Your preamp is a very nice rig.

Alignment is the other critical issue as noted above.  The Pro-Ject system works well but as you said, one really must be patient with it and double-check everything.  The one I have (Strobe-It) is quite a bit thicker than the average record.  You don't say which arm or specific tool you have, so you'll want to see where your VTA is both on vinyl and the alignment tool to make sure that isn't an issue as well.

Good luck & happy listening!

I'm not surprised that you found adjusting VTA made a noticeable difference that brought you closer to your goal. IME, it's a super important adjustment; possibly the most important. Personally, I will only use tonearms with VTA adjustment on-the-fly capability. I tweak it for every LP I play with superb results.

That said, I still feel a cartridge that's a closer match to the preamp phono sections would be the way to go.  At a $2K potential budget for a phono stage, you can get a truly outstanding cart that matches either the MC or MM gain on the C100 (a high end rig by any measure).  Without another box between you and the music, BTW.

Even better would be a second hand Graham or Micro Seiki arm with VTA OTF and any one of a dozen <$1K LOMC or high end MM cartridges to mount on it at the same $2K overall.  That approach kills 2 birds with 2 stones (so to speak), and gives you a more flexible platform for the future. I also live in the sticks, and that is a key consideration for me.

Good luck with whatever you decide. Keep us posted and happy listening!

@uberwaltz 
Yes; I adjust for every side of every record.  My system easily resolves the difference and it only takes a moment.  The effect is substantial and very much worth the effort to me.  I recognize that not everyone is that picky or sensitive and that's just fine.  To each their own.   

I'd had a Rega with a LOMC for years and it just sounded like I was missing something.  I then went to a Magnepan Unitrac arm with the same cart and a whole new world opened up for me. That was in 1984 and I've never looked back. 

The OP just described the same effect after doing some more adjustment, so thought I'd share my experience on the point.  That's the joy of this hobby: Getting to happy listening. 

IMO, the whole confusion between VTA and SRA stems from the fact that it is not possible to adjust one independently of the other.  FWIW, arm manufacturers describe this adjustment as VTA while a corresponding SRA adjustment feature in a cartridge is not available.  Regardless of what the geometric adjustment is called, the idea is to get the stylus to the correct vertical relationship between its edge profile and the grove of the record.  While the -2 degree specification that Soundsmith insists on is a good place to start, the cutter heads on record presses may or may not hold that geometry.  That's why an audible difference can be observed even between the 2 sides of the same record.

The issue of VTF is also a concern because it is affected by VTA.  Physical adjustments have to made (e.g. changing VTA by removing the mat) to accommodate for the height of the scale tray relative to the combined OAH of the mat & vinyl.  This is necessary to obtain a reasonably accurate VTF measurement for actual playback conditions.  Every tonearm is a bit different, but a long-standing rule of thumb is to get the VTF set when the arm wand is parallel to what will become the vinyl surface.  That will minimize VTA (inclusive of SRA) errors due to variations in vinyl thickness as well as minimizing induced VTF changes.  As the OP has discovered, it's also a very tedious process in fixed VTA arms.

These points of geometry are precisely why I am such a big fan of VTA OTF capability in tonearms.  Setting VTF becomes a rote task.  VTA (again, inclusive of SRA) becomes something that can be unambiguously controlled in real time.  It sounds better or it doesn't.  Stop adjusting when it sounds as good as you can get it.  The best arm and cartridge combos can resolve even minute differences to drive profound results.

Not everyone is willing to address all these minutiae simply to play recorded music, but the effort is worth it to me.  That's why audio is such a great hobby!