Ok, I've had a lot of experience with different fuses here. I will agree that the direction of the print on Synergistic fuses does indicate the direction (at least it sounds better to me). The same goes for Furutech fuses. Of course, Hi-Fi Tuning and Isoclean already have a directional arrow printed on the fuse. I have found that when the fuse is placed in the opposite of the preferred direction, the midrange has a weak / hollow sound to it and the overall sound does not have as much impact.
The Audio Magic fuses are just the stock "Little Fuse" brand. They are decent industry stock fuses, but nothing special (like $4 a fuse). Audio Magic drills and fills the fuse with an anti-vibration solution, which acts to reduce electrical resonances. Resonance definitely plays a part in sound quality, but I have never tried an Audio Magic. Also the Little Fuse wire element is nothing special (just tinned copper at best). These were just too expensive for me to audition.
In my opinion, the choice of fuse really has to do with what direction you want to take your NAD. From what I have read, NAD equipment can be very warm sounding. This can come across as somewhat muddy or slow (similar to tube). If you want to make the NAD more clear sounding and quicker/tighter, then I would look at the Synergistic SR20 or the Furutech. The Furutech is the "fastest" sounding fuse I have tried. It will really increase the punch, authority, clarity, and cleanliness of the sound. The SR20 is the second most fastest fuse.
SR RED is probably the most non-intrusive fuse I have found. It will make every piece of equipment sound "ok", but it doesn't make it "sing" (I think you actually lose some resolution with SR RED). My experience with Hi-Fi Tuning Gold put a metallic etch on the sound. I have not tried other Hi-Fi Tuning fuses or SR BLACK.
My favorite fuse is the Isoclean, which is also the cheapest. It is great for solving problems with very bright "solid state" type equipment. It adds a fullness and level of warmth to the sound and takes away a lot of the brightness problems. The Isoclean works wonders in my equipment, but I use very strong and bright "solid state" style equipment (such as Krell, Bryston, Emotiva). I have found the Furutech and SR20 fuses do not actually work out as well in this type of equipment because they make the equipment too "solid state".
Depends on where you want to go. You can get Isoclean on ebay for $30 direct from China, and I'm sure it will improve things, but the Isoclean may not be fast enough for your tastes and equipment.