Making advance seat reservations for Eurorail pass

Passholders often have to pay an extra fee for their seat reservation. That seat cost is built into a point-to-point sale -- another reason that many passes aren't worth it, in either price or nuisance factor. A much more learned discussion than this can be found at www.seat61.com

The way to check is to contact whoever you plan on buying the pass from and ask them to check about the specific time/date and what the associated cost of the reservation would be. I think that you will find that point to point will almost always be cheaper if you are having to buy a reservation on the TGV in addition to the pass.

Contact the sales agent and get specific information about reservation availability on the train you are interested in. If it exists and you want to buy a pass, do so right away and get the reservation.

The main advantage of a pass is almost never to do with price, but with flexibility. It sounds as if flexibility is not your priority and you have specific plans.

Check seat availability at https://ricksteves.mytraintravel.com/. You can put a few reservations into the same shopping cart with the rail pass, if you're ready to. Reservations are now only sold to match the class of service on your pass, so you must compare 1st- and 2nd-class reservations separately.

Make sure you are checking prices on the correct websites. For the Paris to Barcelona train, use French rail https://en.oui.sncf/en/, and for your trains within Spain, use Spanish rail http://www.renfe.com/EN/viajeros/index.html.

Since you say you know your travel dates, you should not have a problem buying advance non-refundable tickets. These tickets will be discounted, and most likely will beat the price of a pass, at no more hassle (by the time you buy a pass and all the reservations, you will have expended as much effort as just buying the tickets themselves).