By "older" I am guessing you may mean a "classic" type of spring powered or SSP type of match air rifle?
If so, I agree with 18 wheeler - any of the classic match air rifles can be regarded as "superior" by a fan of any particular type - but as far as accuracy goes there is probably very little to choose between any of them.
The real difference will be when you find a rifle that you feel the most comfortable with - and that will vary from shooter to shooter as everyone has their own ideas about what they prefer or look for in a rifle.
The Model 66 is an excellent match rifle but as it has a break barrel action it may not be the best suited to your bench rest discipline - but then again neither would an SSP type either as both require lifting from the bench to cock and load them.
For your choice of disciplines I can understand why you suggested a side lever rifle would be an "upgrade" and I agree that the FWB300S would be a good choice - likewise the Anschutz 380, Original Model 75 and Feinwerkbau 300SU types are all worth having a look at and taking a few shots with them each to see what appeals most to you.
Let us know what you end up buying and why you prefer it to your Model 66 as this is always interesting to fellow classic rifle shooters.
Rossendale Target Shooting Club. Every Tuesday and Thursday evening 7 - 10pm.