Pioneer 5250 + Infinity 9633i, not satisfied with SQ

Mohan, have the speaker wiring checked - if you don't get good bass it means that one speaker is wired out of phase with the other (usually between the left & right).

However we don't know (and cannot tell) what kind of sound you're looking for. Therefore it may well be the case that everything is fine but you don't like the sound (where others might like it). In such a case you have to try out different speakers (and maybe also HUs) and choose the combinations that sounds best to you.

I am also having the Pioneer 5250 & Infinity 9633i but i am fully satisfied from that. Sometimes me also feels some jarring noise on low frequency bass.

Infinity 9633i is not a bass maker (woofer). Infinity speaker gives the best quality sound output in vocal. The clarity of sound is awesome same as you says home theatre effect but worst when it comes on low frequency bass. Check the phase also. It also effects the bass. If u recently serviced the car it may passible that some water or mositure goes to speaker(suffer this once). Remove the speaker from parcel tray put them in reverse direction & play them for few minutes. Due to some dust they may jerk.(suffer this once).

9 X 6 speakers can give good but not the decent one. 

First of all add a good 4 channel amplifier (preferbly JBL or Kenwood) with minimum 50 RMS per channel. If u need a decent bass add a free air subwoofer on parcel tray with mono amplifier.

Yeah, coax speakers are susceptible to dust build-up in the voice coil air gap (the centre, that is) because it is open to allow for the mid/tweeter. But since we have to mount them facing up because of the mid/tweeter. My speakers too were rendered nearly useless because of this - until I took it out, removed the mid/tweeter and vigorously cleaned the gap. As good as new, but this time I put them back with a thin (but fairly dense) cloth on the grill to prevent dust entry.