I have a GK 150MBE combo amp that I really like for its portablility. It sounds great when I plug in an extension cab that extends the limited frequency response of the speaker included in the combo. Would replacing the speaker in the combo give me a better sound when not using an extension cab? I was curious as to if there are any replacement speakers on the market that would work in a small sealed box and improve the performance of this combo. Anyone try this?