Vinyl and subwoofers

The original description was "vinyl sounds muddied."
While all the comments about setup, amps, etc. may have merit, the fact that it appears only on vinyl means - well, it must be associated with vinyl playback.

Two possibilities:

1. Acoustic feedback
2. Eccentric recording.

If it’s acoustic feedback being picked up by the cartridge, fix that.
But more likely is #2.

Almost all records have some warpage, which is amplified at 33 1/3 rpm, or 0.5 hz. (33.3 cycles per second/60 seconds per minute = 0.555)
Plus there are various imperfections in the surface. Say there are three imperfections - they will be played back at 1.5 hz.
OR, many records don’t have the spindle hole punched exactly in the center - another source of 0.5 hz.

In all of these subsonic cases, the amp will be pushing or pulling at very low frequencies, which will stress it. And the woofer will be struggling to produce an in-phase signal while the woofer moves in or out, with large excursions.
The simplest form of speaker distortion is the Doppler Effect, but there are other phase-shift effects that will affect the sound you hear. Muddied is a good description.

That’s why amps of vinyl vintage had subsonic filters, usually switchable. And they usually acted below 10 hz. Simple solution (if the slope of the filter didn’t introduce too much phase shift of its own).

In the vinyl era, rumble filters were different from subsonic filters. Rumble filters were designed to mask bearing noise, which occurs in the audible range. Using them eliminates audible sound - better to get rid of noisy bearings.

Subsonic filters were strictly for eccentric recordings. You could actually see the woofer moving in and out at 33 1/3 hz - I thought it looked like they were breathing.
Different problems, different solutions.

You don't need a rumble filter.....  Did you check if the cartridge is mounted correctly...that the mass of the arm is proper for the cartridge compliance, etc. 

I'm almost going to guarantee your problem is acoustic feedback from the sub. I know, because I had the exact same problem, and the way you're describing it was exactly how I would describe it. (And my turntable was also on a very stable surface, a granite countertop.) The way to know for sure is if you have another source besides analog (like digital), if the muddiness is there with both, then yes you may need to look into proper EQ for the sub, but if it's only the vinyl, then its feedback.

The solution is simple: get a 12x12 (or 18x18) stone tile from any building materials store, and put four sorbothane feet on the bottom. Put your turntable on top of that. For me, the bass boominess and rumble disappeared immediately. I have a lot of second hand vinyl, and I never see a rumble issue unless the record has serious warpage. Improper setup can also increase rumble. I have no filters in my system, and it plays everything just fine (even with subs) after doing the fix above. Good luck!

Since you are experiencing this with two different turntables and multiple cartridges, acoustic feedback and physical set up in the room must be part of the problem. Can you try wide frequency response head phones capable of very deep bass with the speakers and sub turned off to see if you get the same muddy bass?

That said, my first thought on reading about your problem is that you have a cartridge-arm compliance mismatch as stringreen suggests. I have a sub that can make noticeable pressure in the room below 10hz, and I am getting real sub bass/woofer pumping problems with certain high compliance cartridges on my medium mass Thorens arm playing certain vinyl recordings. Note that I never get this with digital recordings. Turning off the sub or listening to headphones that drop off below 20Hz seriously reduces the "rumble" with vinyl, but since I am using the sub with monitors I miss the bass extension with the sub out.

My solution is going to be to experiment with cartridges with very different compliance ratings to try to find the right match with my arm. Maybe also trying a phono amp with sub-bass filter. Curiously two different Grado cartridges with the same compliance ratings have noticeably different susceptibility to this rumble problem on my table.

The first thing that should be addressed is the compatibility between the ARC PH8 and the Strativari cart?

There was never any mention of the OP's room dimensions, or what, if any treatments? Subwoofer placement or any info regarding this. Room treatments. For that matter, the rack's construction...?

For a small room, especially, I find my Vicoustic Super Bass Extreme Panels to work very well. Others have mentioned GIK. They failed to say that these treatments work best in a closed environment.

I can only assume that since the OP stated the TT was 4' between each speaker that the Op's set-up has the rack in the center of the room. The rack... we aren't told what sort of rack, only it's 5' tall.

My fix? Ideally, your tt should not be in the center of your speaker placement. I can only assume that you have limited options on your system /room by either your WACF or your budget. To eliminate the effect of the center position of your rack in your room to a more appropriate place.. this would be a cost factor that you don't want or need? Right?

Try the springs from Machina Dynamica to place under your tt support/platform. A very modest investment. A very important/relevant return for that investment. This will be the best option in terms of value for the dollar you will likely spend in any instance.

Good luck!