am playin an ibanez rga121.mostly riffs.i invite suggestions as to which combo amp should i use for a tight,well defined,bottom-heavy sound.something like the contemporary bands- godsmack,disturbed,etc.

Get something from Mesa/Boogie. Not a Lonestar, but something like the Rect-O-Verb or Dual Rectifier Roadster. Both are expensive, but you could probably find them one eBay and they will be the sounds you are asking for.

For a cheaper option, a Peavey Transtube might be able to do it with a good pedal or equalizer brought into the mix. In fact, the best way to get these sounds on a budget would probably be to grab some kind of modeling amp (such as a Line 6 or a Vox Valvetronix) and add a great deep distortion pedal (such as the BOSS metal pedals (metal zone and metal core) or the Electro-Harmonix Metal Muff).

These suggestions are kind of generic, I admit, but they will work. Good luck, rock on.