You should see a tiny bit better separation with monos. Where the inprovement comes is in the separated power supply. You now have more amperage delivery to each channel. The most audible change would likely be in the bass region, that and micro dynamics.
That said, the gains would be relatively small compared to your Golden Tube. If Radii is using the same Chinese PIO caps, I'd bet your GT sounds cleaner than the stock Radii. Personally, I'd do some mods to the Golden Tube. Coupling caps, bypass caps on the power supply, nice resistors in the signal path, power film resistor in the B+ and if you can swing it, Black Gates as the main power supply caps. All of those or a combination, will make a drastic difference in the sound of your GT. If you can't solder, give Bill Baker a buzz at Response Audio (he has a Circle here). Bill does mods all kinds of gear.