good monitor speaker for a large room

In my limited experience the Krell Resolution 3 is the most dynamic monitor i have personally auditioned and has very good bass. (updated nodel is the Krell Res 4) That being said it is still a monitor with limited extension and you have a large room, so it will sound as such. What are you trying to accomplish? Do you have a sub?

Revel M20. You will not need a subwoofer. Very highly reviewed. You can get them used here on Agon for a fair price.
Many reviewers use this as their reference monitor.

Dynaudio special 25's (used) if you can find them.

Forgot to mention with the S25's you really want the stand4 to attach them. I would also fill the posts with sand to weigh them down.

You may want to consider the Totem Mani-2. With enough power driving it, it will fill your room with solid bass. No need for subwoofer.

Active Event Opal with built-in near 500wpc of total power each plus room correction pots beats with price and performance models priced 5x as much.

No doubt different speaker and stand combinations present different requirements for optimizing their synergy. So don't just assume that heavily mass-loading the pillars of stands will dampen the bad stuff and give you the best sound. Peter Comeau, the U.K. loudspeaker designer associated with lively sounding designs at Heybrook and Mission, as well as the (apparently) strongly reviewed recent budget offering from Wharfedale, the Diamond 10.1 [I believe] recommends, instead of sand or lead shot, to use KITTY LITTER. No kidding. The idea here is that all that mass leads to energy storage and its delayed release, smearing the sound and compromising the PRaT factor.

So I'd suggest bagging that sand/shot so you can cleanly experiment. Something like 6-8" of sand & shot mixed, plus a fill of the kitty litter might -- just as one example -- give you an ideal combination of stability and speed. That energy storage factor could possibly be exacerbated by having to fill a larger room from stand mounted boxes.

It would seem that a lot of mass would allow energy release from the speaker. The driver is moving in and out to push a sonic wave forward and into the room.
The more solid the platform (mass), the more energy released from the driver.
I'm sure there are speakers that might benefit from a low mass dampening material such as kitty litter.
I'm not going to empty my stands to find out.

you must investigate and try if available the Mark and Daniel Maximus Monitor+. It should be a great match for your space and listening requirements. http://www.audiogon.com/cgi-bin/cls.pl?spkrmoni&1318733858&/Mark-And-Daniel-Maximus-Monitor

Well, just remember, ALL the energy you pump into those speakers has to go somewhere (and your acoustic wattage is flea-powered by amplifier standards): it doesn't just magically dissipate and disappear into damping material, as some seem to believe (I suppose from too much "education" by equipment reviews, and not enough education in physics). Z-man made a different point, I believe; but if your stands are rockin', I'd say you need new stands! Putting much extra mass high in the column is not likely to usefully increase the stability of a proper speaker/stand unit -- more likely, the opposite. I'd load up the base plate, or secure the whole deal to a more stable substrate as necessary, if stability is the issue. Experimentation is called for, especially if you are changing that structure substantially with added mass. Don't believe uncritically stand manufacturers who say casually, go ahead and load it up: That's called marketing to conventional expectations. Once they have the money... it's YOUR problem.

In looking more closely at this issue, most audiophiles with speakers from the "Recommended Components" list (Chinese menu?) will be HEARING less than 2% of the energy delivered from the amplifier. But also heard is the the electrical reactance and the mechanical resonance (forms of energy storage) of the system -- in interaction... just when you don't want to be hearing them. You'd like excesses of these NOT to be launched into the room. With vented/reflex speakers, given typical alignments, this delayed energy can be substantial. A stand which itself is a reservoir for stored energy could be thought of as rather "full up" with vibration. In effect, the energy gradient between speaker and stand is shallower, and, recalling your education in thermodynamics, therefore the system has less potential to do work -- in this case, drain energy away from the speaker cabinet, and especially, its interface with the drive units themselves... most audibly when this is needed most.

Both that interface with the drivers, and the interface with the stands' top plate will be subject to chaotic interactions, a mix of constructive (not in a good way) and destructive reinforcements. Wishful thinking about mechanical diodes won't help you much here. So the issue is not just one of low level smearing, but also the production of other distortions and the loss of information. All of this jumbled energy is quite divorced from the timing of events the voltage source (your amplifier) is trying to impose on the loudspeaker terminals, thus impeding the sense of agility and flow in the reproduced music. Even PRaT-deniers may not like the result. Keeping your stands from overtly "singing" is addressing but a fraction of the problem, and in many cases, may be exacerbating more serious areas of harm. I would surmise that something like this reasoning, conjoined with his long experience and access to the results of advanced measurement techniques, is behind Peter's recommendation. Newton's law of action-reaction must be addressed; but maybe not at the expense of opening up too many other cans of worms.

Thenis, are you seeking an honest reliable rendition of the signal or a small monitor sized speaker that is easy to listen to? Thanks

In a room that size a monitor speaker might get lost. You may want to consider a subwoofer. I can't imagine anyone not being happy with a REL.

I purchased the Special 25s because it did a good job in a 17ftx21ft room open on one side, and with a high power amplifier had very enjoyable midbass for its size. For deep bass that you can FEEL, you will probably need a subwoofer. My four post Sound Anchor stands are very stable with these.