If I end up going with the M80ti's, will going with a tube amp be better than a SS amp? I currently have a HK PA-5800 amp and the specs are 80 watts into 8 ohms or 110 watts into 4 ohms, both from 20 Hz to 20 kHz with 0.03% THD

So will this be fine? I do plan to purchase a nice 2 channel amp in the future and have this 5 channel amp run the 4 surrounds and the center channel in a 7.1 setup. The local audio store has a Niles SI-250 SS amp for around $250, its 125 x 2 8ohms, 185 x 2 into 4ohms and 395 bridged mono into 8ohms. Anyone familiar with this amp or can suggest a great (inexpensive) 2 channel amp for the M80ti's?

"Better" is a subjective word.
Will this equipment with this speaker sound better than that equipment with that speaker is a question that can only be decided by a single individual.

Tube amps are reknownend for adding some colour to the music so it really depends whether you are searching for neutral clarity or a coloured, possibly more robust sound (for lack of a better word).
Just offhand it sounds like you already have a very capable receiver, so i would suggest to start with what you have, then sometime down the road begin to bring home other components to audition and compare. Then you can decide for yourself whether you like the tube amps over SS or not.