To my experience upgrading current rectifier with better parts is quicker, smarter and cheaper approach with larger outcome. Just modifying for tube rectifier seems to be more like spiritual masturbation.
Tube rectifier maybe more safe for especially power tubes as it rises up operational voltage substantially slower than semiconductor.
In preamp you may be better off with solid state sonically. There are upgrades to SS rectifiers you can research for more quiet designs with higher quality semiconductors and for a lot less money. I've done in the past replacements of separate diodes by integrated low-noise bridge on tube power amplifier. It's available in Newark.com and look for proper parameters of Semicron bridge models specifically designed for tube electronics with high operating voltages. Also another cheap trick is to throw over the AC terminals Jensen film in oil cap of few thousand pf -- these you can get at partsconnexion.
