Subwoofer choice, Sub to pair with Sonus Faber

I have a pair of Sonus Faber Concertos in my bonus room that I will be relocating to my loft so that I can do some more serious listening with them. Currently I have a very old Velodyne 10" non-servo subwoofer running with these speakers. The Velodyne is way to slow to do these 2-way speakers justice. I am considering either building a sealed sub around a Morel driver or purchasing something that is ready built. What subwoofer would you recommend to pair with the Sonus's ? I'm trying to keep this as affordable as possible while doing the Sonus's justice.
REL Storm or Stratus.
I have heard Martin Logan Descents paired well with SF speakers.

What is it that you hear that prompts you to say your current sub is too "slow"? It's possible that your setup is the culprit and, if so, dropping a newer sub in the same location will likely lead to the same results. I suggest you try to measure the in room response and level matching between the sub and mains just to see what your setup looks like. With that info you may decide that EQ is really a requirement and that will drive your purchase.
Take a look at "TBI Audio Systems" subs. I have a pair of the magellin VI's and believe me they are fast and would work well with your Sonus Faber Concerto's. I use mine with Sonist Concerto 2's and cannot tell where the subs come in to play, it just sounds like my main speakers have added bass. They have an outboard amp crossover that you can hook up one or two subs to, although when two are hooked up it is still mono, that is why I bought two of their amps so I could run stereo bass.I run an extra pair of speaker cables from my main amp to the subs high level input so that the flavor of my main amp comes through the sub amps. Check them out! They do come up for sale on the gon every once in a while.Good Luck with your search.
What Bob said.

I've managed to successfully mate Maggies with subs (a GIANT pain in the ass) only with the aid of effective EQ - in my case a (app $500) Velodyne SMS-1. Precisely determining level match and optimum x-over frequency by ear is beyond my capability. An EQ system is -IMHO- virtually a requiremment for obtaining the best sub/main speaker integration possible.

The other great benefit of bass EQ is the ability to correct the inevitablely destructive room effects below 100hz. If the SFs roll-off high enough in frequency, you can EQ the major problems below the x-over frequency (that is, the line level signal to the subwoofer) without inserting anything into the main signal path. If the SFs extend too low in frequency for this set-up, you may also wish to use an active high pass, like my $300 NHT X-2, which will minimize intrusion in the main signal path.

The difference before/after is dramatic, and audible not only in the bass where EQ has been applied, but also in mid-range clarity where it has not.

Good Luck

Thanks for everyone's feedback. My "slow bass" issue may lay in the room setup as well as the old entry level Velodyne. I am going to treat the room and investigate the EQ options before laying out any additional monies on a sub. This way I can be sure that everything is in an optimum state and I will have a better idea of how much a new sub setup will help.

I don't know what the ML Decent sub goes for used but new it is a bit out of my price range. The TBI stuff looks interesting but the TL design, as intriguing as it is, scares me a bit for music. Don't TL designs have phasing issues? I suppose I should listen to one first before thinking it won't serve the purpose at hand. How about the Sunfire subwoofers? Is their sound more geared up for home theater or do they do music well?
I have installed two 2x4' 6" thick bass traps in the rear corners of the room along with two GIK 242 Elite panels in the front. I have also repositioned the sub between the front speakers.

The combination of these changes, evaluated one at a time for effectiveness, have improved the quality of the bass substantially. The bass response is much smoother and sounds much less "one note" than it did before. The definition of the bass is much improved. However the old sub still sounds "boxy" to me and I think the passive radiator design is just not suited to what I want to hear.

The bad thing is wife has demanded that the sub be out of sight so I will probably have to procure a smaller sub and try to hide it behind one of my future main speakers. The good news is that she has also demanded I move away from monitors on stands to floor standing speakers. Since I am only a two channel guy the sub will only be used for home theater. I am not quite as critical of my HT applications as I am of my musical presentation. I should be able to try one of the small TL designs or sunfires as a result.
Glad to hear that dealing with the room and sub placement led to an improvement. You haven't said how large your room is...a 10" driver just won't make a whole lot of clean bass.

You might consider a 12" driver in a smallish sealed box like the SVS SB12-Plus.

Not to beat up the Sunfire subs (they are just too small), but if you research their measured performance on the web you'll find that they are a poor choice for HT -- distortion goes through the roof quickly.