Upgrade to Dedicated Preamp

I have a pair of Paradigm Studio 100v4 speakers, an older Parasound 2 Channel Power Amp, Marantz SA-8004 CD Player (net yet purchased) and a Yamaha RX-V2500 Receiver. I use the receiver as the preamp in the system. I am thinking of getting an older preamp for around $1,000 to replace the Yamaha. Would that be a noticable upgrade in my system?
It's hard to say. If you don't need source switching, you might consider an external DAC with volume control instead of a preamp.
yep... the preamp idea is based on more than one source AND not all of them digital.

If that is the ticket for you here, I'd say you are on the right path to increase performance levels.

Really soon hereafter I'd seriously look into a way better source, or bit true playback via a personal confuser/server arrangment.

in all, I'd say yes.. a noticable gain... How noticeable? That's the real question here. A nice tube preamp would be more of a noticeable change IMHO than a SS preamp on the same level.

I suspect here that the preamp idea is but one step of many to upgrade or repalce outright your current system.

If so, you are headed right in the right direction.... the only caveat I'd mention apart from the initial one I m& Bob made here, is this; Better is always better. if mulitple sources is the goal, I'd add for $2K - $3K (in the preowned) market, there's a lot of killer preamps. past that range, one has to jump way on up to do better. But the $1K preamps are easier fare to best by adding only double or so thhat amount to begin with.

Last thought here is this... think about going balanced or sE now. It'll pay off later on, especially if you wait and drop the 2-3K for one now.
The Yamaha was never intended for high end 2 channel. Mediocre theater at best.
A very simple unit with gold plated jacks and high end volume control should help.
I have always had a combo 2-channel & HT system. I used to listen to my 2-channel sources through my Sherwood-Newcastle P-965 pre-pro and Odyssey Audio Stratos HT3 amplifier. Adding a dedicated 2-channel preamp (first a used Conrad-Johnson PV11, recently upgraded to a used McIntosh C220), has made a large inprovement in the sonics of my 2-channel chain. Although the Sherwood is a pretty good sounding pre-pro for the money, I hear improvements in soundstage dimensions, bass extension, timbre, image definition and detail. While Yammi AVRs are a good value, I doubt they can compare to a decent dedicated 2-channel preamp. I would recommend you look for a tube-based preamp.

For a $1000, you should be able to get a used, but minty Conrad Johnson PV-11 or PV-12. I am sure there are others as well, from companies like Rogue Audio, Cary, Audio by Van Alstine, Prima Luna,Audible Illusions, Jolida, Audio Research, Belles, Manley, VTL and Quicksilver. IMHO, most any of these preamps will take your system to another level.