1. The value can be changed but IMHO you should keep the value between about 3500 ohms and 10,000 ohms. Below 3500 ohms and above 10,000 ohms, the character of the VTA ST-70 and ST-120 will change.
A. Below 7500 ohms will increase feedback, lower the gain of the amp but if you go below about 3500 ohms the amp will sound "harder" and less "tube like". The lowest I have ever gone was to 2200 ohms and I did not like the way the amp sounded - too hard, less mid bass ..
B. Above 7500 ohms will decrease feedback, the gain of the amp will go up slightly, the bass seems to get a little sloppier above 10,000 ohms. The amp sounds a little more "tubey".
2. You can also remove the feedback line by just removing the wire that goes to the two "NFB" eyelets on the front of the driver board. Beware, however, because there are some issues that could arise if you do this. The amp MAY now not be stable with certain speakers. If the amp "motorboats" (low level cyclic noise) with your speaker or exhibits any other abnormalities, you better reconnect the feedback line. I haven't had any troubles with the speakers I have here but the load that some speakers present to the amp could cause problems for the amp (without feedback). Another issue is that without any feedback the gain of the amp will rise. The VTA ST-70 and ST-120 sound warm and fuzzy but not particularly accurate without any feedback.