Subwoofer insight.

This is new territory to me. Current speakers are 2 way with frequency response of 60Hz-20KHz with +/- 2dB.  Sensitivity rated as 86dB. Chance to purchase a pair of REL 7 tis. I feel like I am missing something but will have to buy the subs to try out. My basic question is this. Does this purchase sound wise? I know what I am willing to spend but do you think I will notice an appreciable improvement.  I know that no one can listen through my ears but this is my first time doing this. Source is 75 watt integrated tube amp, McIntosh MA-2275. Thoughts appreciated. I am leaning towards going forward with the deal but would appreciate some feedback. Room is large and open, 28X38 feet with 9 foot ceilings. Thanks for any feedback.
ATC SCM19 has a published spec of
Frequency Response (-6dB): 54Hz-22kHz
Even 1000w is never going to get low end from it or any similar 2-way.

Power requirements fall approximately 50% per octave. Using the SCM19 as an example and rolling the bass @ 80Hz requires an amplifier of only 50w vs 200w full range. The smallish woofer is relieved and more easily accomplishes its task as a mid -bass & -range driver. There is no free lunch and no 6" driver is ever going to have the same output at 20Hz or even 40Hz as at 200Hz or 2kHz in any box of reasonable size.

A purpose built sub with 300w RMS peak has double the power necessary for the 20-80Hz range as the 50 for 80-20kHz. Something in the 10-12" range with similar sonics to the mains & properly set up will add musical realism beyond any amount of power.

IMO, unless one has completely separate HT & music systems, one should opt for musical sub[s]
REL is going to reproduce the subtle spacial nuances in both music and home theater in ways that level up your experience and your system as a whole. As Ricred posted earlier, he's had a number of different subs and ended up with dual REL Carbons.

I have another friend who bought a REL Carbon as well and liked it so much he added a 2nd. 

There's a reason you see RELs with 2 channel high end systems at the shows.
There's a reason you see RELs with 2 channel high end systems at the shows
where some of mostest, awfulest sound on the planet is purveyed.

IMO, any sub w/o a continuous phase control, polarity invert and multiple slopes is a toy.
There's a reason you see RELs with 2 channel high end systems at the shows.
Better profit margins for the dealers?
There's simply a level of nuance in spacial reproduction that the other's don't match. There is much more to a sub-woofer than going thump really loud at 20 hz.