Brad Johnson said:
Well, as you say, the results are highly variable. It would depend on the particular pickups and the design of the particular amp. AND, obviously, the design of the active preamp. That's usually one of the most important factors. A "well designed" active preamp won't change your sound. But, there are still changes that occur "after" the preamp, like for instance in the instrument cable. Various cables can make a HUGE difference in the sound that results. The same cable that works well for an active instrument may sound horrible with a passive instrument, and vice versa. That has mostly do to with the transmission line properties of the instrument cable, relative to the source impedance, and the load impedance. In a poorly matched system, you may be losing a big part of your midrange signal, without even necessarily realizing it (as you say, having nothing to compare it to). Someone guided me through this experiment some months back, and I was totally amazed by the results. They astounded "even me", and I've been doing this bass thing for about thirty years now.
Here's what I'm saying: in a "well matched system", where all the pieces (pickups, preamp, cable, amp) dovetail, you can probably flip switches and tweak knobs ad nauseum and never suffer any significant dropouts or undesirable sonic changes. However, in real life, "most" systems aren't well matched. Most systems are built up piecemeal, and most salesmen don't even know what the concept means. Consider "hum" for example. You can take a bass that's perfectly quiet with one amp, plug it into a different amp, and it'll hum like crazy when you're walking around the room. I've done that thousands of times in my career. Usually that kind of thing happens with passive basses, just because of the antenna and transmission line properties of the pickup-cable system. Active basses are less prone to that, although the system still obeys all the basic laws of physics, but the source has the property that it isn't "as much" affected by the varying impedances along the line, so the sound "appears to" be more consistent. But, switch to a different cable, and the chances are good you'll hear some changes in the sound.
With passive instruments, you're more in the risk range, just because of the way the pickups are connected (ie one end is usually grounded), and the higher source impedance. The grounding thing is especially insidious. With a passive bass, you'd have to run a balanced line (like, and XLR mic cable) between the bass and the amp to address this problem. With an active bass, you just use a differential input preamp and you're home free. Why more manufacturers don't do this is beyond me. Les Paul knew about this issue, and countless others. But the industry has standardized on single ended connections, hence we often have buzzy systems.
What saves us, strangely enough, is that many systems aren't "good enough" to hear the difference. But take your bass into a studio and use an Avalon Vt737sp with it, and I guarantee you'll hear the difference. I'd bet a year's salary on it. I've heard it with my own two ears, and the results are absolutely consistent, it happens the same way every time.
Bottom line, one must design to one's ear. It doesn't make any sense to try to achieve "technical perfection", 'cause that may or may not relate to what one hears, and whether one thinks it's pleasant sounding or not. There's kinda two different things, there's the "tone" of the system, and there's the "operation" of the system. In a perfect world, those two would complement each other and the result would be synergy. In the real world, there's sometimes a tradeoff. For instance, in the real world, I'll take a quiet hum-free instrument over one that has perfect tone. Because, tone is subjective, and when you're in the heat of a gig, few people really pay attention to it anyway (except for the other bass players in the audience). But, hum is noticeable by everyone, and no one thinks it's pleasant.
I was trying to use everything available while still cutting the mix too. I did a lot of soley P bass and a nice mix of the J pup too. I got vast tonal variations and was very pleased, but as I said no one else even noticed. I must choose good tones because I always get raves about my sound if it's not drowned out by the kick. Any way I'm very happy with all my basses and it's hard to choose which one to play half the time. 
