I tried a full set of EH's, and then a full set Tung Sol's in my Mesa .50 Cal +...and feedback was not a real problem with either. I live in high-gain territory. Your experience may vary.
Tonal differences were more apparent. The Tung Sol have more bottom, and carries it's smoothness to the top. EH's are more harsh, and "in your face!", with less definition (more crowded sounding). Perhaps the "toppy-ness" is the reason for more squeel people report?
I compromised, by putting a Tung Sol in V1, and a balanced Tung Sol in the phase inverter position. EH's are in the two other preamp positions. Tung Sol's in V1 are hard to beat, and my Mesa's power section likes them at the phase inverter.