I doubt if it is the tubes.
Maybe the tuning?
Those sweep tubes are very touchy as far as resonance.
I don't have a 590, but I have listened to one.
I think it sounds very good, but I would put an external audio amp and
speaker on it.
That helps a lot.
Its not just the dynamic range, the quietness of the receiver is also
critical.
The other thing you will enjoy about a new radio, is the ability to know
exactly what frequency you are on, and how to get to a chosen frequency.
You will get used to no tuning the drive or amplifier very quickly!!!
I went kicking and screaming from a C line to the ts-480.
I have no idea why I waited that long!!!
If you run CW, 50HZ band width with out ringing is also a feature you will
enjoy.
This is all not mentioning frequency stability, which allows you to listen
for hours with out adjusting a knobb.