Not sure if "turning up the preamp" refers to onboard the guitar or amp? Either way with a hollow body this is without a doubt the reason for your feedback issues. Adding the TPC1, unity or non, will not change the EQ nor the feedback issues you are experiencing with your current setup. The buffer gives what it gets, so if the guitar is feeding back you are just going to open up the high end and get feedback with more clarity. Taking a guess at possible remedies, i would replace any pickup height adjustment springs with some rubber type. Next i would try and raise the pickups, this will substantially increase volume. I would also try increasing the amp volume versus the preamp volume. Just a couple things to start or look into with just the basics of your rig. I always try and workout the clean tone first.
Disclaimer: I make and sell Buffers. I also sometimes Modify, Build and sell preamps and amplifiers. My opinions are not intended to be sales pitches. I am a one man shop and prefer to spend time with my wife and family, i work full time on call and love to spend my spare time doing other things. I only make, modify or build things for those that seek them.