Will upgrading my amp for el34s to kt90s help?

The Atlas in Magnum mode is far better than the stock model and for only $600 more the power supply is substantially bettered as well as resisters .the EH kt-90
tubes are excellent now the amp has balls and finesse .
I like the sound of this even better than the stereo90
which is $500 more, also the caps are Mundorfs which are excellent German caps and bypassed with a 1% tolerence caps and the transformers are of super high quality ,Please remember you have 8 and 4 ohm taps take off the cover and try this for impedance mismatches of speakers make a world of difference especially on a tube amp ,also with the Atlas each tube is individually biased
and it is a snap with the excellent meter in the middle
meaning if a tube goes you no longer have to buy a pair.
For $1800 The Rogue Atlas Magnum is far better built and sounding than any competition at this price and looks way retro cool doing it .There is no overseas amp that is built this well, looks this good with sound to match
at anywhere near this price ,The Magnum is the way to go !!

I agree on the Magnum. I have the Rogue Cronus Magnum as of about one month ago. I'm actually having the opposite problem you're having. I have a little too much bass. Prior to this I had Rotel separates and I had almost NO bass. The difference the Cronus Magnum made was more than significant. Rogue is a fantastic company. You should call and speak to Mark O'Brien.

It should. EL34's are not known for their strong, very present bass or low end grunt at all. They are a midrange and treble tube mostly, that is why so many guitar players use them. A bass player, if using a tubed amp, would use KT88's or 6550's in most cases. Those tubes are designed for higher current draw and have inherently stronger and more dynamic bass (presence of it and punchiness). KT90's are variants of the KT88's and should be a good improvement.