At what vol level do you get rumble / flutter?

Twice your normal listening level? Turn the goddamn thing down. It's an inexpensive table and it can only take so much.

Tobes,
What would you suggest in situations where the tonearm/cartridge match looks perfect on paper, but the woofer pumping continues? I have a VPI Classic with a Lyra Delos and I still experience excessive woofer movement. The Classic stands on a 4-inch thick maple block, which in turn rests on Isoblocks. The rack stands on four heavy concrete squares and there are a few more in the rack to add mass. Granted, the turntable is between the speakers, and only a few feet away from one of them, but this is the limitation I have in the placement and it cannot be altered.

I'm just curious what your advice would be in situations where there is no apparent mismatch between the arm and the cart, you can't experiment with placement and you've done nearly everything possible to address resonance short of moving to a different apartment or spending $$$ on a state-of-the-art rack or replacing the analog front.

Actusreus, I don't have any personal experience with Lyra cartridges. I did notice in looking at the specs of the Delos that the compliance is quoted as "12 x 10-6cm/dyne at 100Hz", indicating that the compliance would be considerably higher at the cartridge resonant frequency - but this in itself shouldn't push the resonant frequency too low with a typical medium mass arm. It's actually difficult to get a really bad match. Even the highish compliance (for an MC) AT PTG-II posed no issues for me in the medium+ mass Phantom - though, anecdotally, any issues I've had in the past with other arms were with higher compliance cartridges.

I have read that Lyra cartridges are demanding and put a lot of energy back into arms. What you are getting may be the combination of the Delos' slightly higher compliance (= lower resonant freq), strong transmission of energy and the perhaps the VPI arm is not ideally coping with this. Have you played with the oil damping on the VPI?

Assessing the resonant behavior with a test record can help you see what you're dealing with. I've found there can be a marked difference between what the specs suggest and practical results.

I suspected the tonearm might be at least partly to blame. I did apply a little bit of the damping fluid a while ago, but will experiment again. Thanks Tobes.

Actusreus, you're finally on the right track with the damping fluid. As some have mentioned before, your problem is not an issue of wow, flutter, or rumble. Rumble comes from noise from the motor (usually an old style idler drive turntable) getting picked up by the cartridge. Your problem is most likely a mismatch in cartridge compliance and effective mass of the tonearm, which causes the woofers to visually "pump" at a very low (e.g., 10-13 Hz). Damping fluid can lower the amplitude of the resonant peak. Increasing your tonearm's effective mass can lower the frequency to where it has less audible effect and improves cartridge tracking.

Here is the cartridge/arm compatibility for the Ortofon MC-3 Turbo. Its compliance is 13 and cartridge weighs a very light 4.1 g. The Pro-Ject Xpression tonearm has a low effective mass of 8.7 g. This puts the resonant peak at 12 Hz and close to 13, which is outside the ideal of 10 Hz. If you could add 3-5 g to your arm's effective mass (which you could do with a headshell weight), this would push the resonant frequency down to the 10-11 Hz range which is near ideal. According to the Xpression's specifications, the stock tonearm counterweight can accommodate cartridges weighing from 7-12 g and there's an optional counterweight for lighter cartridges of 4-7 g. As you can see, you have one of the lightest cartridges on the market. Did you switch to the other counterbalance to accommodate it?

Since you're better off sticking with the original counterweight and increasing your tonearm's effective mass to lower the resonant frequency, get a headshell weight, heavy mounting hardware, or both to increase the tonearm effective mass by 3-5 g to compensate for the low weight of the cartridge. This will increase the effective mass to overcome the relatively stiff compliance of your new cartridge. If you do this and top off your damping fluid, the woofer "pumping" should diminish significantly.

You may also want to consider an aftermarket turntable mat to help dampen resonances and further isolate the turntable's mechanical noise from the cartridge. A record grip or clamp may help a bit too; it certainly does on my rig. It doesn't have to be expensive--the acrylic one from Clearaudio or the rubber one from KABUSA.com (both around $30-35) should do fine.