If you feel up to emptying your wallet Jim I think that amp should be able to run 5998s on the output without modification and they should sound quite a bit better than just about any 6080 variant you'll find. The 6080, in my experience, is a pretty tubby sounding tube. Another "leaner" option would be the 7236, but again I'd have to scour datasheets to make sure you wouldn't be overloading the heater power supply.