Scott, I assume you are talking about the venerable PL400, and not the improved PL400 Series II. The difference in sound between a stock PL400 and one completely rebuilt with White Oak Audio (WOA) parts is phenomenal. The posts above have merit, many think the PL400 is not worth rebuilding. However, I have done the full PL400 rebuild many times and I can tell you, IT IS WORTH IT! Most people do not need more than 200 Watts per channel anyway. With the rebuild you will be getting 250-300 Watts per channel. The difference in sound quality comes mostly from the new Rev G1 Control Board that replaces the original Phase Linear control board that has a faulty design, poor layout, and components that are 40-50 years old. The difference in increased dynamic range comes from the new WOA main Power Supply Capacitors with almost triple the reserve power capacity. The difference in lower noise and distortion comes from the new WOA Backplane Boards that eliminate the vintage point-to-point wiring on the back of the amplifier. The difference in sheer power that will knock you over comes from the 16 new Output Transistors that have much more gain and are more closely matched due to modern manufacturing methods. You really need a pair of decent speakers when you get this upgrade.
I really like the sound of the PL400, it is smoother and less shrill than its big brother, the brute, the PL700. I have both and I can listen to the upgraded PL400 all day; the upgraded PL700 (1000 Watts) is perfect for scaring animals and small children, after a couple of hours listening to live thrash metal bands like Metallica, AC/DC, etc. I'm done.
But its your call...
