Jokerman1226,
I can confirm that a 5U4G will *not* fit under the cover of a stock LS100. I believe Dan can provide a cover with a hole to accommodate taller rectifiers. I ran a 5U4 in that preamp for a long time, and I just kept the cover off. FWIW, my favorite in that preamp was the 6106, which is the same size as the 5AR4 so the cover will fit.