Larry, I would check the specks before subbing in 5AR4s as these are not a direct substitute for the 5U4s. In my experience with the SLI-80 I would stay away from 5U4GTBs (note the 'B'). These are the straight sided tubes with the higher rating, 275ma vs. 225ma. They made my amp too hard and bright. I also like the Syl. 6SN7GTAs from the 1950s. These are the box plate , not the angled plate versions. What will really make a difference is finding driver tubes with really well matched triode sections. I test mine on a VTV Dual Triode Tester. My favorite 6922s are the Amperex 'white label' PQs. If these are too bright you can try some 1960s Mullard 6DJ8s with the steel pins. They CAN sound wonderful; but are prone to noise and microphonics. If you use them you might have to buy and test a few to get a good pair. Good luck and have fun! JT
