IMO, for you, the best solution would be a multi-band equalizer.
I would start with an inexpensive one, if happy with the effect, keep looking for one that fits your system best and perhaps higher quality
If nothing else, you will have learned which frequencies need taming for you, that helps when evaluating models of cartridges/cables/phono stages/any component you might consider in the future.
Does your preamp have two outputs? Put eq between preamp/amp after riaa eq has occured.
Do you have a tape loop to try an equalizer in/out of circuit?