Best way to jam-a-long

[KKND]

Hey there, i usually play along with backing tracks but ever since i got the stage 60 the 2 5" studio monitors don't seem to blend well with the amp, if i get the amp up to my preferred loudness, the M-Audio monitors get all harsh and growls and if i set the amp a bit lower on volume, the speakers are just too loud

Mind you i don't have a proper USB interface to play the monitors they are being played through BOSS GT-10 which is not ideal...so my question is..
a)if i get a proper USB interface M-Audio or Line 6 will it help?
b)do i need bigger speakers
c) any suggestions on placements in the room?

And finally, how do you play along, through amp speakers???? i always wondered if that's is recommended!!

Your input will be appreciated guys