12 inch subwoofer overkill for desktop setup?

First of all, why do you think you need to replace your current sub?  

What kind of volumes do you listen at? 

I have a nearfield (not quite desktop) system with Harbeth P3ESR about 4 feet from my listening position and a 15" 725 watt subwoofer on the floor right next to my desk.  My room is about the same size as yours.  I listen at moderate volumes and you'd probably never know the sub is there unless I was listening to some EDM. 

It doesn't "overpower" the room.  These things come with volume/level controls that allow you to integrate them with your speakers.  I just have a lot of headroom and that sub will never be strained in any way.  It's barely "idling" if you were to compare it to driving a car.  It's definitely overkill and I could get away from something smaller.  I previously had a less powerful 12" sub in this room and that also worked well.

I'm a proponent of multiple subs (I have 4 in my main system), but sitting that close to the speakers and sub, there's no need for an additional sub.  Having multiple subs in that small space will have little, if any benefit, unless you are stuck having it in one spot and there is a big peak or null right where you're sitting. 

There's also no need to "relieve" the speakers of the lower frequencies.  First of all, they only go down to 75hz, and I'm not driving them with a flea amp.  In another system, that might help, but not in mine, and without knowing more about your system, nobody can tell you that's necessary.

I don't know anything about your speakers and sub, but they look like they are designed for home theater use.  What's the rest of your system?  What about it do you currently find lacking?  What do you like to listen to, and how loud?  There are probably other things to consider than the size of the cone in the sub if you want to improve your system.

@soix pretty big, 1.8 m length table.

@noromance don't think I will delve into multiple subs. Too many cables. 

@big_greg @elliottbnewcombjr The Riga was bundled with Santorin 30-500 sub when it was launched a decade ago, the way Cabasse intended it to be listened. I'm using the smaller Santorin 21, my DAC is Lab12 Dac1 Reference and my amp is Sony TA-A1ES. I think my system is already pretty good and as nice as a few of the showroom setups I've been to, but hey, I will make small upgrade here and there whenever the fund allows.

The Rigas are satellite speakers and as such the sub has to take care of frequencies below 100hz. They are well, really excellent for listening to music, but also excel for home theater. Think of slightly livelier ATC SCM100 and you will get pretty good idea how the Rigas sound. The Santorin 30-500 comes with built in microphone and calibration kit for its DSP, so perhaps it should be much easier to integrate into smaller room?

Wont overpower the room with proper placement! Keep the crossover point as low as possible. 

Sounds like a nice system for the room and if you enjoy it now, I'd ask again what you think you need to "improve".  You mention integration.  Do you feel like your current sub is not well integrated with your speakers?  Do you have room to experiment with placement?  Have you experimented with the crossover?  Phase?  If your speakers only go down to 100hz, have you tried a higher crossover (assuming you can)?  It's unlikely that getting the bigger sub is going to have any detrimental effect, so you can always just get it and try it if you have the budget.  You might consider a sub that allows you to crossover higher and has additional controls for PEQ, like Rythmik's subs, allowing you to manually configure the sub to your tastes.  The ease of use of the DSP option is nice though and could be just what the doctor ordered.

@big_greg Honestly I am hard-pressed to think what to improve in my setup. My recent DAC purchase provided that final bit of upgrade that finally brought my setup to sound as equal as showroom setups costing easily 4-5X more. So from now on, I understand that any upgrade I make is probably only going to improve my setup marginally. I have not experimented with crossover, phase, and placement yet, but maybe I should. I followed Cabasse's recommendation in the manual to the letter on sub placement and crossover setting and they seemed to work well.