I'm getting and have a bad lower back. I've got over 100,000 miles on motorcycles and have been through the various styles. I'm no expert, but can share some thoughts based upon my experiences.
When I had the Electra Glides, 500 mile days were routine if I had the crew to do it or if I was by myself. I cannot overemphasize the importance of handlebar and footrest position(s). I think these factors are even more important than the saddle though I would never ignore the benefots of a good saddle. When I was younger I had a 650 Yamaha and did nothing to it except highway pegs - what a difference in extending range with comfort.
When I got the 850 Suzuki I put an aftermarket
cafe' racer fairing on it and realized that I could ride longer with less fatigue. When though the phase with the crocth rockets and found them to be surprisingly comfortable but did not offer enough flexibility of position for the longer trips.
Then I got into Harleys and never looked back except for a great BMW Enduro bike I had in the early 80s. The cooler looking my Harleys were, the less comfortable for the longer trips they were.
I no longer do 500 mile days because of time constraints and too many other interests. I will travel again when the family is older and given todays choices I would likely get a Road Glide (frame mounted fairing

)and a Corbin seat with backrest. I would want multiple positions for my feet so that I can alter/relieve stress on my lower back. Vibration absorbing grips and good clothing will make the road a pleasure. In as much as I only get out for a 100 or 200 miles with the guys now and again, I ride a NightTrain (95 c.i., 10.25:1 forged pistons, cams, headwork, 42mm Mikuni, ThunderHeader, bumped rev limiter, etc.

) and really like it; but it would not be my first choice if I had the time for road trips.
Spark, if you are uncomfortable, you need to look at riding position. Perhaps the current adjustments are not right for you or the style of ride you have needs to be reconsidered. Try to ride as many different types of motorcycles as you can. Get with a group and ask to swap for an hour here and an hour there. Brands are not important, it is the style of mount/rider position that matters most. Then you tweak with adjustments and multiple pegs and what not. Lastly, a quality saddle that supports your back and spreads the load over as much ass as posible without pressing too hard against the aft section of your inner thigh (common problem when not-so-wide people buy so-wide saddles).
Happy Hunting.