@R.G. Would adding the silicon stop the Rectifier from SAG? It looks to me as that is the only reason they put the tube one in was to get SAG on the power tubes
I get a lot of flack on this issue from die-hard tubes-rool guys, so I'll blather on about it for a minute.
First, "sag" is a fairly complex thing. Like everything complex, naive users want a simple, easy to understand solution, so they pin their hopes on the first explanation they like.
Sag comes from several things, but the one most often mentioned is the power supply drooping because of high signal outputs. This happens because with heavy loading, the resistance of the power transformer and rectifiers means the filter caps can't be refilled as fast as they're being emptied, so the voltage drops some. This causes some of the characteristics of audible sag by lowering the output power of the power stage and lowering gain in the preamp tubes a little.
Compared to tube rectifiers, silicon rectifiers have negligible internal resistance and forward voltage. So they not only provide more voltage at the filter cap, they don't lower the voltage under heavy loading.
If you have tube rectifier, it will have a tens-of-volts offset and noticeable internal resistance. If you have only silicon rectifiers they will lose 0.7 to 1.2V per diode, and have maybe 10 ohms of internal resistance. In general the voltage at the filter caps will be about 40-60V higher and will not sag because of the rectifiers, although voltage ripple caused sag will be the same, being controlled by the size of the cap.
If you use only silicon rectifiers and a resistor to fake the voltage drop of a tube rectifier, you can get a near-perfect match to a tube rectifier. Weber sells this solution in its "copper cap" rectifier replacements.
If you add silicon rectifiers in series with a tube rectifier, it will make essentially zero difference to the amp - until the tube rectifier fails. Then it saves your power transformer and filter caps. Adding a silicon diode this way does not take away any sag the tube rectifier puts there.