A better quality Selector Switch makes a big difference. The Volume Control could also use a better quality type. After break-in, you may want to experiment with the signal capacitors. I used Vcap Oil's with very good sonic results.

Of my GG modding I would rate them in this order, the first two being definatively noticeable, and fall into the category of "you get what you pay for".  Those after 1 and 2 are subtle and an opinion at best, but would add those Mods are going through Beast amps into high effciency horns, and in front of that is the Phono Pre with a good MC cart. (and a balanced power supply), so the system is resolving enough to hear minute differences.
1. Attenuator.  I've used the kit/stock, Alps Blue Velvet, Bruce's stepped (series), and now a mono ladder type Elma for each channel (which also gives a little balance control too).
2. Output caps.  V-caps oils, and I really like Mundorff Gold/Silver/Oil.
3. Clean Power: adding Bruce's balanced power supply can do wonders. In addition I also employ Power Line modules/Filters in place of the stock IEC.  This could be a local thing and not repeatable at other locations.  I go for power as clean as it can be had.
4. Connectors, Selector and Signal Wire: I plug and unplug a lot so the gold plating wears off quickly on connectors unless I use a higher quality (Kit connectors are fine otherwise).  Cardas GRFAs.  Elma selector (which matches Bruce's stepped and the Elma Monos) and a shielded twisted pair wire from the RCAs to Selector switch.
5. Passives: resistors, electrolytics and diodes.  I use PRP resitors and Mills on the higher wattages.  I am convinced the resistors used in the signal path make a difference.  Low ESR caps on the power supplies.  For Diodes: FREDs.
6. Tubes?????  Running them at 12 volts (versus 12.6), it doesn't seem to make a lot of difference, as long as they're a quiet tube.  (Would like to try a curcuit with 317/variable Voltage reg to see how it affects the sound).  I've tried lots of different tubes and would suggest one spend money on attenuators or caps before trying different tubes.  Gold plated sockets to prevent corrosion, high temp o-rings for tube dampers, and DEOXIT Gold on tube pins and Interconnects/RCAs.
7. For scratch-built I can detect no difference between a toroidal and the stock type trannies.  The toroidals just look cool.
8. Feedback values.  I keep coming back to the stock 100K value.
9.  I use a silver coated solid core copper hook up wire with Teflon insulation and a quad eutectic solder, or something with a little silver (and lead) in it, both just a personal preference.
10. Decent (not expensive) interconnects.  Something that's shielded.  Currently using some Dayton Audio interconnects from Parts Express: not expensive, work just fine and have cut down on the iterference problem I was having.

Did I miss anything?

Modding the GG should yeild more results than modding the upstream or downstream components. 

And, as a DIY hobby, this is still cheaper than vintage car restoration..............or a visit to the dentist.......... or lap dances down at the Badda Boom Badda Bing.........