Hi Donut!
IMO, this sort of set-up can work extremely well and best of all appears to offer good value for money, especially if you are not willing or do not have the space for very big floorstander and bigger amps (or even an array of amps). My main motivation for having such a setup was to throw lots of power in at the bottom end to be able to really "drive" the room as my listening room is relatively large and open, and has bass suck out problems below about 40Hz. The other was to tune out or to avoid aggrevating a 50Hz room resonance in the room, the main speaker's response drop off at about 55Hz, and the sub comes in at at 42Hz, such is the ability of the room to really amplify the 40-60Hz range, I really don't need to have flat output in the range.
I'd say that there are a few issues to watch out for if you fancy looking into the mini monitor and 2 subs approach:
1) HEIGHT: The tweeter on the main speakers should be roughly of ear level height when they are perched on top of the subwoofer, or roughly the same height as a pair of floorstanders. Mine are just a smidgen lower than ideal at present, but should be sorted out soon when I get some better isolation to go underneath the subs such as a paving slab. The subwoofer's spiked feel are already sat in spike cups which raise the height off the floor a light, as well as some ceramic floor tiles (no shown in the picture in "My Kit").
2) ISOLATION: Most decent subs will have a farily inert cabinet, so mechanical interference between the two shouldn't be much of an issue, but it's still important to get the two halves isolated. You could try balancing the main speakers on isolation cones or suitable sized granite plinths (and blu-tack). I think you maybe just be able to see from the picture in my "My Kit" page that the mini monitors sit on top of something, which in turn sits on the sub's cabinet - that something is a polished piece of granite which came with my speakers as they were originally designed to work with matching purpose made passive bass units as a floorstanding combo - see picture below.
3) CROSSOVER FREQUENCIES: In a way, this requirement means that this solution might be a better mid price to "lower" high-end solution than it would be for someone on a tight budget. I think those on limited finances would be better served by getting a suitable floorstander, as IMO for this solution to really work the main speakers really neeed to go down fairly low, to say at least 50-60Hz, and then for the sub to take over form there downwards. The more auidible range the main speakers produce the better. Also this puts less of a onus on the sub(s) to produce more highly or tonally distinguishable bass which will help with tonal integration. The subs of course should produce as little of their own tonal character as possible, again something which will necesitate a highish expenditure, and for the money, I think the MJ Acoustic Reference 1 I have does a surpringly good job for the money and matches well the £6k mini monitors they're used with!
Setting the levels on the two subs shouldn't be any more complicated than if you had one. The easiest thing to do is to just make sure you have identical crossover and output level setting on both sides. Mine have indentical settings for both sides. Experimentation is an option of course.
As for whether this solution is really the way to go, I'm convinced it is for me given that I don't wish to get bigger speakers and a bigger amp to provide the muscle to "drive" the big room that is my listening room.
I've been told that I should be able to get at least 80% of the bass I'm able to get from the subs, but by going back to using the mini monitors with their partnering passive bass units, with no associated hardware changes, just a change of cables and isolation. Can't say I'm totally convinced by that, but the challenge is on and I'm sure you'll see some reports coming up within the next few months. Yes, I agree and think that I should be able to get the obvious tune playing stuff to be able to play rock/pop tracks satifactorily, but I have doubts whether the floorstanders will have the inner detail, subtely and grace for acoustic and classical which contains a lot of important ambient and harmonic details below 40Hz, IMHO. Below 40Hz is a real problem in my room as it gets sucked out, hence why I use the power of the subs to bolster those frequencies. Also, I can get the bass ports on the mini monitor's dedicated bass units to chuff on one or two torture tracks I have, so reason to suspect they are not up to the demands of the job.
Others also argue that floorstanders are better for timing integration, but I think for my generally more "round" leanings, I think the sub/mini monitor combination will be more than good enough for my requirements.
The only other consideration of course are the aesthetics. For me, this is a non issue, but for others...
fits in well with a 30 grand system
