Any recommendations on Pre-amp
Sounds like a possible fixer
Search the 1500 Tech forum. IIRC there are several threads regarding "amplifier" (not
pre-amplifier) upgrades for the I/S. Bear in mind that a power figure of 60-80W/ch is realistic for the stock alternator/battery arrangement and you'll need to upgrade the fairing speakers too.
I may upgrade the amp and speakers on my I/S at some point but always ride with a helmet w/ headset. OEM stereo is plenty powerful for driving a J&M.
My '99 Tourer was a slightly different challenge. Of course, these come with no radio whatsoever and just a windshield - not a fairing. I had a Memphis Bat Wing on my first Tourer for a while, albeit with a set of Fats windshield mounts and it just didn't quite sit back far enough. I bought the Valkyrie-specific Trigger Lock mount kit for the setup when I built my second Tourer - but lost the ability to also run a Fats windshield without changing the mounts. Until I studied the problem and found that a modified set of Victory Hammer Trigger Lock side plates work just peachy. (Why Memphis didn't/doesn't offer these is a complete mystery, as it's just one hole per side which needs extended into a slot.)
The Memphis Bat Wing doesn't accommodate speakers, but an I/S trunk does. One is on the Tourer now, and the stereo (actually a dual-band ham rig with AM/FM BCB reception) resides in the spot where an I/S CB would normally go. Its built-in amp is good for 15W/ch and I can always install a beefier unit in one of the saddlebags if warranted. Audio to and from it is integrated to the bike's Hondaline CB via a Kennedy FRSet-4.
As with the I/S, I always ride with a headset-equipped lid so hearing the radio isn't an issue. The trunk speakers are more for my passenger's benefit.