I would expect that you were using IPSec with the PIX. You mention PPTP now. Assuming you are using PPTP, the PPTP passthrough needs to be enabled on the Linksys device in question.
Some of the older Linksys products had a real problem with PPTP passthrough. A firmware bug caused the device to advertise a window size of 0 on tcp port 1723. There was talk of firmware upgrades to fix the problem, but I replaced all of the ones I ran into, so I'm not certain if they ever came up with a good update for the devices in question. This would not have been an issue if you had been using IPSec.