I gigged with an SVT into various speaker cabinets from the mid 70's till early 80's. I loved it and sold it when bi-amping became the rage.
For the last several years I gigged with a GK1001RB. I was happy with the power, weight, and features, but never bonded with the tone. I eventually decided it was the low end...just not the "right" character. I added a BDDI and that gave me the type of lows I wanted.
That head died in a tragic "glass of ice water falling into the cooling fan" accident at a gig. I replace the amp with an Ampeg PF500.
I am thrilled with the PF-500. It has the tone I was missing with the GK. No BDDI needed. While the GK provided a solid platform, the Ampeg is more of an extension of my bass.
I am convinced that the combination of the passive "boost and cut" Baxandall bass and treble, and the inductor based active midrange boost and cut of the Ampeg is responsible for the Ampeg voice. I've gone over the schematics for the PF-500 and I don't know how they squeeze that bit of grind out of it.
The schematics for the GK1001RB show a pretty conventional op amp design. The "boost" knob looks like just a master volume on the schematic, but it does hit the next stage harder to produce the "growl" that GK is known for. Simple circuit, good to great results.
In the 70's the band and I went through a bit of an adjustment period, when I switched from my SS Carvin amp (which was a tight, substantial bass head) to the SVT. I experienced the same adjustment period with my current 3 piece, blues/roots rock band. The tone of the Ampeg fills the room differently than the GK did. I am far more satisfied with the voice of the Ampeg than the GK. They are both solid, differently voiced, valid bass amps. I will repair my GK but it will be a backup.