Sound in T-series players is excellent. As Sesquash noted, sound quality is subjective. Much is dependent upon the particular ears involved.
Some claim T-series players don't get as loud as the older ifp-series players. I don't know how loud those people like to listen to their DAPs but the T10 and T30 get plenty loud enough for me. Listening too loud damages ears, which means less hearing sensitivity, which means many things may not sound as good as they actually are. It's a vicious cycle.
The American T-series models do have more output at 18mw+18mw vs. 15mw+15mw for the T-series players available for the rest of the world.
If you want more technical specifications on sound quality check this thread:
T30 Freq. Response: Excellent
The tests in the above thread were actually done on a T30. But I own both T30 as well as T10 models and although there are differences in size, features and navigation, sound is basically the same in both.
As far as availability at offline stores, you may have a problem. These days it's all about iPod and Zune at the bricks-and-mortar stores. The only iRivers I have seen lately on physical store shelves are Clix models.