No one is ok with selling DERIVATIVE works. Modifying samples is not transformative (from legal perspective). To be transformative you have to make a significant change to the meaning of the work, generally-- there's a bunch of tests-- but generally just editing stuff is not sufficient.
Example would be take a play or movie about one thing and 'transform' it into a sculpture that ALSO now means something different, like you are either inverting the meaning of the original work, or maybe it took place in the past and now you're turning it into a comment about something happening today, etc. That's the kind of threshold you're looking for if you want a work to be transformative.
I'm not a lawyer and I'm not claiming this is the minimum threshold, I'm just saying that that this is kind of the intended use of transformative exceptions to copyright, and if you're not doing something like this you have a harder time arguing transformative use.
Recently there was another case where Dr. Seuss sued a Star-Trek themed riff on Oh The Places You'll Go! and they were able to argue transformative use even though it was still a book. So yeah there are always exceptions, but one thing about the law is every case is different. There may be something deep down in the text of the ruling that pushed that case over the line and it may not apply to the next one. Or to yours.
I think the moral question is actually the most interesting one. What does it mean for you to be importing samples and modifying them etc. I think yes you should record your own sounds. I would not import other peoples' samples into Serum and sell them even if they are modified. I just would not be able to accept that regardless of my changes its DNA is still someone else's work.
I would look for CC / public domain recordings and make my own. Especially for wavetables, they are small short samples that don't need huge multisaples/roundrobins, etc., yeah? So taking a few snippets from recordings you made should be easy. Not as easy as just importing someone else's samples of course
but easier than building a huge Kontakt instrument.
But that only applies to stuff I would sell. For my own musical use, everything is fair game. Even Omnisphere (which a lot of people object to its EULA because it limits non-musical use of the samples) says straight up you can do anything you want if you're not selling samples.
What if it's just for my own use and I don't distribute the samples I've created?
If you're a licensed user, you are welcome to use your Spectrasonics instruments and libraries to create new audio samples and loops for your own use. The important thing is not to distribute them.