Perhaps you coudl try out teh Rega Vulcan sub. It would blend well with your current set up and would give you that extra oomph you're looking for.
My suggestion is to work on power, power conditioning and/or power cords. The Mira is small enough that you can run it through a conditioner without current limiting becoming a problem. In my experience, dedicated lines is the highest return on investment, followed by a conditioner, and then cords. This will improve bass, expand the soundstage and improve detail, without changing the basic character of a system that you are happy with. There are many good choices; My bedroom system is a Linn Majik on a 20A circuit dedicated to the system, a Tice Solo ($400) and a Cardas cord ($185?), and it makes terrific bass through 6.5 inch ported woofers. You can get major results for less than $1,000.
I would like to second the recommendation for a sub in your system. I once had the Rega Planet and Mira with Jura speakers. For a time, there an REL Storm II in the system, and it helped bass extension greatly. It was easy to integrate and was very "musical." By the way, The Rega Vulcan is made by REL. You could prpobably find a good used REL Strata or Storm for about the same price as a new Vulcan. Used Vulcans don't turn up very often. Either REL unit will be more powerful and will go deeper than the 8" Vulcan. In my exporience, the Rega gear will benefit from power cord upgrades. The stock ones are as cheap as any I've seen. I wouldn't go overboard on the power cords though.
I third the recommendation for a sub. A REL, Rega (which is designed and made by REL for Rega), or ACI sub would be the first choices.
If you like the upper octaves of your system why risk changing them ? Also these subs will take you down to 20Hz and give real bass-drum kick. I would bet you'd be looking at a lot of money to upgrade your amp and speakers to achieve the same.
Subwoofer is the way to go. You'll be set free...I love my Sunfire Artchitectural Subwoofer. 10" single front firing, made for 2 channel listening. It's a shame you don't hear about this baby enough on the 'gon. It's fast, extended, tight, and integrates beautifully with my M 20s. It lists for $1400, but can be found nicely discounted (new) for less than $1400. It's an 11" (45 lb.) cube with a 1200 watt amp. (Sunfire also has the Architectural Signature, which is a 12") If space is a factor, this may do it for you. I compared it to the Velodynes and the comparable Rel.
I plum forgot another great tweak for your system. Couple every component (speakers too) with Audio Points, including the rack you have your system on. If its wood you can get Audio Points that will thread into the legs of your rack. The Audio Points will be a sonic boost to your entire system. Call Robert, at Star Sound Technologies, and he will give you the scoop as to what size Audio Points you'll need. I'm a coupler. Not a decoupler. People mix the two. That's a crying shame. I don't even want to go there, because of the superfluous threads it'll generate. Email me if you, truly, want to be set free.