Stick with the tubes your amp is designed for.
You won't get any more power. You won't extend the bass at all, and you run the risk of destroying your amp.
Filament current is higher with the KT150 over the 4 tubes it could be as much as an extra amp of current. KT150's also require either higher voltage or higher current to work properly. Anybody tells you there was an improvement likes to listen to distortion. They also require a much lower impedance drive or they could go into runaway. They also may not physically fit.
As mentioned, your amp is probably not the problem, but room, speaker placement or characteristics of the speaker itself.