I know I am a little late to the conversation, but have you ever watched the YT channel LowEndLobster?
He did a whole thing where he modded like 5 Sub Ray4 with Pre-amp and pick ups. he had a video for each mod, then did a shoot out where he played them all in the sam video.
Really helpful if you are looking to upgrade.
In all honesty, I think the pick up makes up most of the sound difference.
I upgraded my pick up in my Ray 4 to the Aguilar pick up for $119. The sound was night and day difference and so much better. My friend did the pick up and pre-amp, (Agiular Pre-amp) and noticed that most of the upgrade difference (noticeable by ear) comes from the pick up.
I bet you could get away with adding two Agiular or Seymour or EMG pick ups, which would be around your $300 budget, and you will love the sound so much better.
I think you might in fact get better results this way, because you would be going for high quality pick ups instead of budgeting between the pro-amp and a more mid line pick up.
Just something to think about and if you get a chance, look up LowEndLobster on Youtube and look for any of his upgrade Ray4 videos and his Mint-o-Ray video and his MOLD SMOOTHIE is fun and educational as well. If you do decide to do the package with pre-amp and pickups, he showed some budget ones that he used for the Mold Smoothie, or the Mint-O-Ray (he was working with a budget on those projects) and he got pretty good results staying within the $300 range (but you have 2 pick ups, so consider that, you may really have to cheap out on the pick up if you replace the pre-amp and it might not be as good as the stock pre-amp with some top line pick ups would sound.)
And once you start watching the LowEndLobster videos, you might need to talk yourself into turning him off, like talking to yourself like a little kid.
Ok, time for bed now, turn off the lobster guy, he will still be there tomorrow, maybe even with a new video. LOL
HTH