I couldn't find any info on that receiver. Do you know if it is a Dolby digital/DTS receiver or is it pro logic?
Anyway if you are using a receiver then I see no reason to use a preamp. The receiver does everything the preamp does. Having a preamp in there wouldn't gain you anything.
Are you sure there is a preamp? I thought the GFA-555 and the GFA-2535 are both amps, not preamps
The amp may help you depending on your speakers and the quality of the receiver. Your receiver needs to have pre out jacks to use the amps. What you would then be doing is using the reciever as a preamp and using the amps to drive the speakers.
Hope this helps,