Budget Strategies forTightening up Bass?

its time for diy acoustical treatment..Thick as you can get and not over done.

Rear port, right? Firing into corners...I'd try different schemes of port damping using various plug materials.

My opinion.

As for your speaker cables, I've tried many and settled on Speltz anticables, even over cables that list for over $1200.00. There is a clarity, and open clean window on the sound that is beautiful and incisive on VMPS speakers. Can't comment on other speakers bu the Absolute Sound sure loves them too. You can buy them in bulk, easily attach spades and do it for under 100.00. i have a 10 ft. run and and 18 ft. run because my equipment is now on the side vs. between the speakers. I used to have an 8 ft. pair. You can't tell the difference in the longer cables or in a mismatched length. How many cables can do that number and still be clear as a bell?

I tried moving speakers 4" further away from back wall-- seemed to make a difference

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If your initial distance from back wall was 10", you may now have almost same distance of the front baffle from rear and side walls.
This seems to infringe one of the fundamental speaker placement rules. It may work for you, but it's largely discouraged.

Power cords? Speakers cables?
The Silverline SR17.5s put out some serious bass for monitors. Set up and positioning for this speaker is critical and being only ten inches from a corner is going to render bloated bass.
If moving them out into the room is not an option I suggest you contact Alan at Silverline Audio for information concerning some sort of port plug as noted above.