+1 to the comments above. One of the things that IMO is most difficult about 'slap style' is to be able to effortlessly go back and forth between fingerstyle and slap with no volume gain or loss and no EQ adjustments (except possibly adding a small touch of low end when slapping).
So, my guess is that it is primarily a technique issue, and is the primary reason that so many amps have those goofy 'footswitchable slap scoop controls', etc... which should be avoided like the plague
That being said, the strong percussive nature of slap style will immediately identify issues with amplification (which is at least part of the reason you see players in music stores 'testing' equipment using slap style playing). Using too small of a speaker cab or too low powered amp for your playing context really becomes apparent when you are wacking that E string. If you are pushing your rig right to to the limit with your finger style technique, then the more percussive peaks and wider tone (primarily due to hitting the string closer to the neck or even on the neck which results in more fundamental) can really result in the low end compressing. I've also found this issue to occur with some tube loaded preamps or hybrids, where the gain structure of the pre tends to compress a bit when really pushed... I hate that!
I've also run into this with a few basses, where the low end fundamental seems to compress when slapped. This is rare though, but it can drive you nuts. It doesn't seem to be 'brand or model' specific, but I guess certain pieces of wood can have this issue, and it's a 'slap killer' if you unfortunately run into one of them. It's like someone is turning down the bass control of your amp with every low E string slap. This should not impact the 'pluck' part of the techique, though, and becomes much less apparent as you move up the neck. I doubt that this is your issue.
The best way to test whether it's 'you or the amp' is to play at moderate volumes, playing the same 'lick' or pattern fingerstyle and slap style. You should be able to go back and forth with no apparent change in volume IMO and IME.