Add a second sub or...

Love my current setup with a single Rel sub. I have about an $800 budget. Would the addition of a second (used) sub raise my system to a new level? What improvement, if any, could if expect to hear?
Or should I spend the money on a VPI record cleaning machine (to replace my Spin Clean)?
I believe you're asking for your stereo system but, I'm going to respond in HT. I just placed a third sub in my HT. I had one in LFE, one (speaker line connected) as part of the center ch (making it full range), and NOW one connected through a Speakon cable to the front R/L speakers. It reduced the BOOM effect but smoothed out the blend. I have more natural, impressive bass. The surround is better defined with voices more detailed. I use a single sub in my stereo setup. I would use two if I had them. I expect you to benefit significantly from a second sub.
It would be easier to give advice if we knew more about your system. Best of Luck
Needfreestuff -
Essence Reference Super Gems, Essence Jasper II monoblocks, Musical Design Chameleon Elite Ultra pre, VPI Classic 3, Arcam FMJ 23 cd, and all Intuitive Design wire..Design of monos necessitates wiring Rel Storm III to the left speaker in lieu of terminals of a monoblock. If I were to add a second Rel, it will be wired to the right speaker.
It really depends on your priorities. Cleaner records or a more balanced and deeper foundation for the music. I am sure there are many who disagree with me but IMHO 2 subs should be mandatory for a stereo set up. I have the Spin Clean and earlier the VPI 16.5. Apart from ease of use, the Spin Clean is a great bargain. No static is a major plus for the Spin Clean.
The 2nd sub would be the same model REL ?
Yes - a second Rel Storm III.