bookshelf to floorstanding-real upgrade?

Keep the NHTs and add a nice musical sub...you wont be gaining much bass extension....unless you go with very large floorstander...in which loss of imaging might come in to play...which will probably overide your room anyway...the NHTS with a sub...such as a HSU,Adire, or sunfire will be your ticket...trust me...

I like Phasecorrect's advice, but would also suggest that you get BETTER monitors, such as the new JMLabs or baby Spendors. Several NHT dealers have woefully suggested that the older Super Ones were MUCH better than the newer ones, so I'd recommend that you focus on midrange purity first, and then add a sub for that bottom end. Three-ways are MUCH trickier to get right, and I suggest that floorstanders do NOT start to sound good until the Thiel 1.6, 2.4 or Revel F30 level. JMO

Let me tell you...and please listen...

first and foremost, I suspect that the Paradigms won't do it for you. NHT, in general, makes much more realistic sounding speakers than Paradigm (paradigm fans, don't crucify me).

Now, back to your actual question...as was mentioned already, bigger is not always better. I had a pair of nht 1.3a's and a pair of nht 2.5i's. I recently purchased a Unico integrated to run the 2.5i's. My room is about 14.5x11 or so...

When i connected the unico to the 2.5i's, it sounded great (although i have always felt that they were too much for the room). The other day, i put my 1.3a's up for sale and when the guy came to see them, i hooked them up to the unico (same room). Wouldn't you know it? They sounded AMAZING in my room with that int. amp. They simply complimented the room much, MUCH better than my much larger and more expensive 2.5i's. So, again, matching is of utmost importance (matching components/ matching speakers to room)

This was proof enough to me that bigger is not always better. (this is something i keep trying to tell my girlfriend too!! hahah).

I'm seriously thinking about getting into some higher quality monitors at this point. I think that my setup/ room could really benefit from them. (I sort of got locked into selling my NHT's ....and yes, i regret it now)

The 1.3 was a nice speaker, albeit too flat-bright for most tastes. With a nice tube-amp I can see why they wowed you, though. Getting SMOOTH-freq response and CLONED-PAIR midranges is in my opinion still the greatest task for speaker manufacturers to get right. So at moderate price points you can either get a fine-sounding two-way monitor or a compromised but bassier three-way. We all know this.
An acoustician/speaker designer friend and I built some 6.5 two-ways a decade ago, and I asked him about the shape of the cabinet, and he said that humans seem to favor the sound from boxes that are about the size of a large head. Aside from the physics of freq-resp and cone diameter requirements, 5 and 6" midranges DO seem the most honest, if cabinet-diffraction "cuppiness" is designed out somehow.
Spendors, Harbeths, Rogers, Sonus Faber, the Revel M20 and even the Triangle and other $500 versions all have their champions, and often sound better than the bloated brothers farther up their product line. Getting a small cabinet to behave quietly is a LOT easier than a large one. Hence a tendency to separate "satellites" from woofer "bases" in some high end designs, too (Wilson, Verity et al)....I'm not sure that the CURRENT NHT midrange is any better than the Paradigm ref series, but BOTH are mediocre compared to those mentioned above. Others will also add their favorite Soliloquoys (sp?), or Josephs, ProAc, Epos, etc., for options. Have fun.