My amp is 260 watts-is that RMS? What can i power?

Yes that does sound like your RMS wattage. Sorry your going to need a new amp. You may just be under powering your MTX if it is a 350 watt rms. So depending on which Kicker subs you are going to get will really determin how many watts your amp should be if you are getting say. two 12" kicker comps you will see on the box it says 300 watt peak 150 RMS. so in that case you really need to look for at least a 300 watt amp. it also must be a two channel amp. you must also know how you need to bridge the sub woofers you plan on getting. I have always used kicker products myself and i will never change. all kicker amps have a certificate inside the box that says how much they can handle. which is always higher then what it says on the box. good luck.