8 strings: the info
There a only a couple of companies that make production 8-strings (including mine). Most of them use a multitude of custom pick-up configurations. Conklin usually uses a Bartolini soapbar that has a very smooth response and solid tone. Novax P/U's are very smooth as well. I've seen Bartolini' in Novax as well as some customs. Acacia uses seymour duncuns and Bartolini's.
Many other of the custom luthier builders can aquire special made pick-ups that can be wound to a certain preference. I use custom EMG 40DC's with a BTC control on my 8-strings. It has a VERY meaty response, but with the BTC control it can be dialed up to a number of tonalities. I also run my 8-strings on 18 volts, so the headroom is more prominent leaving the clarity a little more distinguished.
Bartolini's seem to be the pick-up of choice for extended range instruments. As compared to dimarzio, they're not screamers, but ask any guitarist that has used Bartolini's and they'll probably say that the bartolini's far out weigh the quality of the Dimarzio's. I love the EMG's in my 9 and 8 string guitars, but I'm also a biased observer. I do use Bartolini's, but due to the immense waiting time for them, I'll only use them at SPECIAL request. I'll get some specs on the different pick-ups used and that may help you gauge the sound that they're able to produce.