As an owner of the same sub, you are going to have a difficult time getting better performance, with equal aesthetics/WAF, for a decent price WITHOUT increasing the size considerably. As you know, not only are these subs beautiful to look at, but they are relatively tiny at under 13" cubed.
Basically, you would have to spend $500+ to get a big step up in performance with the smallish form factor (SVS SB1000 minimum) but that's for ONE sub, and you will be replacing two. You have over 4,000 cu ft to fill up. I'd be concerned with the ability of a single small, sealed sub to fill that room any better than the two ported 8" subs did. I wouldn't get rid of both of them until you had the funds to add the second sub, so for example you could do a "phased roll out" where you add one big sub and keep one of the Mirages to help fill in.
Another option, depending on your room placement, is to get one big boy beefy sub that you can tuck away in a corner or stash as an end table, where it is responsible for the majority of the output and will be less of a WAF problem (lower visibilitY), and then keep the more diminutive/attractive Prestige S8 by the front of the room to smooth out room modes.
Either way you are out of the scope of the "budget sub" thread.
EDIT: to provide a little data on the last recommendation (mating a big sub for output with a smaller sub for "fill") I played around at one point with pairing the much larger Mirage Omni S12 to my Prestige S8.
The first graph is just the single Prestige S8 (measured at 4 points around the sweet spot) after Audyssey calibration. Pretty flat from 30Hz to around 90Hz but the two big issues are (1) steep roll-off below 30Hz (as you'd expect from an 8" sub) and (2) a huge null between 70-80Hz that EQ can't fix.
The second graph is both subs combined, also plotted at 4 points. You can see several huge improvements: (1) the system is now flat to 20Hz, (2) the second sub cancelled the nulls higher up, so in addition to yielding deeper low end extension now it's also much improved in the upper bass, flat all the way up to 100Hz with less drop-off above that point, and (3) the dual subs greatly reduced spatial variation so the bass will be more consistent.
The moral here is twofold: first, multiple subs make it much easier to calibrate your bass response because they work together to energize different room modes, thus eliminating big peaks/nulls and reducing spatial variation; and second, with some minimal effort at optimizing placement you can effectively blend one big bad sub with one or two more smaller subs, allowing the big sub to carry the load in terms of output/extension and utilizing the smaller subs to fill in the gaps.