I have back problems as well, due to herniated disks. I paid a great deal of attention to weight when I bought my 5-string Ritters which weigh 9 lbs. and 9.4 lbs. and replaced my Fender P, which served me well in my youth, but was killing me after about half a set. Mahogany is less dense than Maple or Ash or Alder, and there are some basses that are hollowed out redwood or even spruce. If you really want light, get one that has chambers in the body. I don't think you must give up the extra string to get the weight down to something manageable. I would definitely NOT recommend the Barker bass. Lifting is as much of a problem as playing, IMO. Also, try to get a lightweight semi-soft case. That makes a big difference. As far as amps go, check out the Ampeg Portabass 800 head and the light weight cabs they make with luggage cart wheels as standard equipment. At set-up and take down time, I consider these a blessing from the gear gods. These are also plenty loud for most applications. You might consider a separate small rehearsal combo that is light weight also. When the back starts bugging you, every chance to get rid of a pound or two of regular lifting is a must. Also do everything you can to stretch out before rehearsal and gigs. It becomes a maintenance ritual. I've lived with it for about 15 years now. Pinched nerves, sciatica, weak knees, etc. You want to do everything you can to minimize strain.