LP playback is lacking - any ideas?

Where do you have the table set up? Is it near the speakers?You could be getting muddy sound because the table is to close to the speakers, don't forget that your VPI has no real isolation. The Glider is also a pretty laid back cartridge but it normally isn't congested. Is it all albums or just some? If it isn't on all, maybe it's the recording. You have nice stuff you should be getting great sound. You might want to check your overhang and tracking force again.

Has it always sounded congested? If so you could have a few set-up problems. The most common is the impedance loading on the cartridge. I almost never use the recommended load--I do use it as a starting point, but then listen. Lowering the load will bring out more bass, increasing it will bring out treble (very generalized). When doing this you need to be sure that the VTA and cartridge weight and antiskate are all set correctly. These will effect the loading, because they too can have similar effects. For example if the VTA is set too low, it generally brings out the bass. If you set the load with the VTA too low, you would probably use too high a load to compensate. The result would yield something with correct tonal balance (to some degree) but would not sound nearly as open as it could. I am not familiar with the AR PH-3--is it a tube phono stage?? If so, you might consider new tubes if they are getting on in years. I have recently retubed my Sonic Frontiers phono stage with NOS tubes, and it really made a huge difference--probably should have done it a few years ago.

The retube on a ph-3 is dirt cheap - $40-50 or so. The tubes live 2000 hours. I would go to someone other than ARC for the tubes. They are (sovtek?) standard 6922's. ARC really overcharges for their tubes these days.
On the topic of cartridge loading: The ph-3 comes from the factory with input impedance = 47K ohms, input capacitance = 90 pF. Your ph-3 should have come with a set of resistors that you have to solder into the ph-3 to change the loading characteristics. Your ph-3 manual will have a table of resistor to loading. If you don't have this table I will post it for you. An easy thing you can also try is to look at your line stage preamp and see if it has any inputs that take a lower input setting. My ref1 preamp's aux1 input was 3 db lower than the rest of the inputs. I found using this input worked better for my phono stage. Good luck.