Weird issue with my VPI... please help

You could have a bad cartridge. I bought a Benz L2 which gave me fits trying to zero it in. A test record showed that it would not pan which seems like your problem as well. Anyway I sent it back to the factory for inspection and they concluded that the coil windings were faulty and replaced the cartridge.

The only way to know is to check all the setup parameters and if it is setup correctly and you still have the same problem, I'd talk to the Dealer about checking the cartridge for defects

What you describe is NOT a VTA issue. I don't even know how you can set the VTA to get what you are hearing. The cartridge may be connected out of phase. Easy enough to check.

OK, here's some more info.

When I first noticed how weird it sounded (absent midbass etc) I quickly tried another phono stage - a Pro-ject Tube Box SE II and in about 30 seconds assessed that it didn't sound much different (leading me to suspect the cartridge or turntabe setup).

This morning I tried the Tube Box again with the Aerosmith LP and it played the 'missing lick' just fine; also noticed the soundstage was much wider than the Rhea. Going back to the Rhea it pretty much collapses to the center. If I really listen closely, I can hear the missing lick, but it's normally mixed very forward. You'd think if it wasn't playing one channel it would be obvious (e.g. the image wouldn't be centered between the speakers). So there's something wrong with the Rhea.

But it doesn't stop there exactly. While it sounds better on the inexpensive Tube Box phono stage, it still doesn't sound at all full - like there are dips all over the place. It's not the speaker or preamps because If I play a CD, the frequency spectrum is fine.

Any suggestions?

Starting to really sound like a bad cart. Could have an intermittent coil connection... I'd contact the dealer.

Must be a wiring issue within the cartridge.

My experience is that Shelter performs absolutely outstanding in a VPI TT, so stick with it and isolate the problem.