Set the volume at the lowest setting at which the woofers pump with the platter spinning and the stylus in the groove. Place the arm at rest. Does the pumping stop or continue? If the woofer pumps with the arm at rest (at a volume at which you might play music), the cause would appear to be acoustic breakthrough/feed back loop: TT/arm/cartridge > amp > speakers > air > TT/arm/cartridge > etc). The solution is to minimize acoustic breakthrough via one or more of the following:
Increase TT to speaker distance
Place
TT and speakers in different rooms
Increase
TT mechanical isolation: if TT couples to a raised wood floor, site
the TT on a shelf mounted to wall studs
Change
TT from unsuspended to suspended (I prefer unsuspended TTs).
Change
to a TT that better resists acoustic breakthrough (Sota's old Saphire
and Star both may have set the standard for being impervious to
feedback, but Sota motors are small and low cost, and the TTs made
audible W&F....I use a 68 lb TT: Empire 208 motor/platter/bearing
on a 1.5” solid alloy custom plinth ala the one Atma-Sphere used to
make and sell).
It appears other users reported good results with your arm/cartridge pairing.
I suspect the pumping would be visually similar in your application, even if you swapped speakers (as long as all were reflex loaded). The reason is that reflex loaded woofers (mid bass in your case) unload below the port resonance, and for all full range speakers the pumping resonance is well below the port tuning resonance frequency. (Even if the pumping looked similar between 2 different speakers, pumping likely audibly degrades one speaker more than another, based on many variables.)
But suppose 2 persons employ the exact same arm/cartridge: person A's preamp/amp cuts off very low and makes high power in the bass (your OTL amps make more power @ 16 Ohm than 8 Ohm, the opposite of typical SS....also I don't know the speaker's impedance at pumping resonance); person B's preamp/power amp cuts off much higher and has minimal bass power. Person B's results may be fine, with little to no pumping, while person A's woofer pumping may audibly degrade performance. This could explain why your pumping is worse than another system with the same arm/cartridge.
Unfortunately I lost the link, but in the 70s IIRC, a European PhD. (I think a major European company like Bruel and Kjaer employed him) did a scientific paper on the subject of the ideal resonance range. He concluded that the ideal resonance frequency range is significantly higher than usually recommended. I use a Pioneer strain gauge cartridge, high compliance and very heavy. When I lowered the Rega RB300 arm resonance with a lighter counter weight, performance audibly improved by huge margin.
Long term wise a different arm/cartridge pairing may be the best solution. My strain gauge needs a true low mass arm, so I purchased a used Audio Technica AT-1100 with two wands. I shall add mass to one wand for use with a mono cartridge.
Your SMEV is in another universe v. my AT arm. The AT arm's effective mass is only about 7g. If/when finances allow, I plan to upgrade to the Moerch DP arm, which has 4 different wands of various mass from about 4g to 12g.
I always loved the looks of the SMEV, and the pro audio reviews were effusive in their praise of its performance. I'd buy one if it fit my needs.