If there's any part for an SVT one can get cheap, one should apply a fair amount of scrutiny. It may already be broken down.
If it sounds dull, have your hearing and/or the amp thoroughly checked. Something is wrong. The simple fact that they are probably the most popular, common, and iconic bass amps should say something.
Popular and iconic is best reserved for Justin Bieber not one of the best bass amps ever made. The fact that lemmings like something is not a reason to like it. The B-15 is niche popular to those in the know but isn’t on every stage but unbeknownst to the public is on many to most of their favorite music from a certain time. Let the SVT stand on its own merits.