what is the best speaker cable?

While recommendations from others are a good starting point, shopping by brand or price can result in a big waste of money. I would always try before you buy. The best source I have found for this is The Cable Company (1-800-FAT-WYRE). They rent cables (Speaker/interconnects)for about 10% of the purchase price and all rental fees going towards any purchase you make from them (they also have equipment and accessories). They carry essentially every brand of cable available, so you can try a wide range of brands and prices. Even the outrageously priced exotics (like Nordost Valhallas)! You may find that a brand you hadn't even considered may be best (as I did when buying inexpensive interconnects for a friend. The Wireworlds I tried were far better than anything else in the price range!).

Hope you don't mind my asking, but I also own the Prima, and wonder what the change to the Telefunken tubes gave you in terms of sound quality? Also, where did you buy them?

I know that almost all brands of Monster are the worst.
I use a short 3'runs of isolated speaker wire from VanDenHul(CV-122H model if I will trust my memory). Before I've plugged them instead of my JPS Superconductor cables, I plugged them into AC outlet with 200W bulb for about 2 hours for real break-in. The result is -- I've sold my JPS cables after I've pluged in these magic wires. I've paid just $60 to get this wire, and I use silver-solder covered bare wires to connect to binding posts.

Among brand names there are only few brands to pay attention for VanDenHul, AudioQuest and Cardas(newtral reference).

Aside from the obvious comment that there is no single "best" speaker cable, it would be helpful to have some additional information:
1. what is your budget?
2. how long will the cables be?

You obviously have good speakers, so you want something better than 16-gauge zip cord (smile). However, based on a number of years of experience in the audio hobby, I have a strong bias against the prices being charged for some of the high-end cables. If your objective is to get good yet reasonably-priced speaker cables, then you should have Kimber Kable 8TC or 4TC on your list. They detract very little from performance -- which is all any speaker cable can do, or ought to do. The lower-priced Cardas cables are also good. If you look back through the Audigon archives, you will find a LOT of threads about speaker cables. I suggest you start by reviewing previous comments -- use the A-gon search engine to find the threads. Ultimately, you should audition ANY speaker cable before you buy it -- how cables perform will vary somewhat from system to system. As with all things audio, trust your own ears. With speaker cables and interconnects, there is a sharply declining performance-to-price ratio.

Cost no object?? Nordost Valhalla