I've played at least two-dozen pre-Gibsons (I own an '87 "Classic") and three Gibson basses from different production periods, and I can see how someone might prefer the latter, though I don't, personally. Aside from some superficial cosmetic similarities, they really are very different instruments.
The basses made by Mike and his crew have a warm, dark, smokey richness, and possess a very characteristic organic low-midrange bark that I find quite unique.
The Gibsons I played sounded big, modern, punchy, and bright, but I found the tone a little nondescript and perhaps a bit sterile. I doubt that I could pick one out of a bass recording lineup, whereas the pre-Gibsons would be pretty hard to miss.
My experience has been somewhat different.
First of all, I have played and owned a bunch of Mike's original instruments; and I found every one of them to be so totally individual that saying that they sound like this or that just doesn't mean much to me.
I currently have #1014. It is very thick sounding; but, that is largely due to the woods involved - mostly walnut.
For a long time, I played and worked with #20XX, a Lacewood Sig 5 built in Burbank. I absolutely could not get that bass to sound like anything at all. Maybe it was the lacewood. I talked to Mike about the bass. He called lacewood the styrofoam of woods. That's about what the tone was like.
In the course of things with that bass, I carefully upgraded to the then current Gibson 18 volt preamp. Before doing that, I had a number of conversations with the Nashville Shop, and found them to be extremely knowledgeable, helpful, and generally on top of their game. To some extent, some of those folks got a bum rap, IMO/E. As things turned out, the preamp was a lot better, but I finally gave up on the lacewood and sold the bass at Bass Northwest.
After a couple of years with Jazz basses again, I decided to get a 6, just for grins and giggles, and purchased a Gibson era Classic 6 sight unseen on eBay. That was me being stupid. But, it was $1500, and I figured - what the heck, how bad could it be? "Shockingly," that bass has turned out to be the best all round player I've ever owned. I've had soundmen, band leaders, bandmates, all kinds of folks just gush over the tone; in preference to my Sadowsky, SR5, Jazz RI, etc. And other than having been played in quite a bit, it has flawless fretwork, excellent construction, etc, etc. It is hands down the loudest acoustical electric instrument I've ever owned. I guess it bugs me a little that the carving on the back of the upper bouts goes straight across the neck transition rather than the graceful curve that Mike put into his; but I try not to let it get to me.
Some of the Gibson work, at least, was not all bad; and not all of Mike's work yielded perfect results. I guess I've learned to assess each bass one at a time. Kind of like people.