This is for Georgehifi especially but others can chime in.

My F5 has about 15.5dB of gain.

Say yes!!! about half normal power amps.
Which is very low for a poweramp, and why your experience with passives is understandable, and why you need the gain of an active pre.
 
As Nelson said: " "We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more."  

And there’s no need for active buffers, on passives if there’s no impedance mismatch.

Cheers George

Your statement " All passives basically suck" is your own problem with your system/amp gain structure. Live with it.

Cheers George

You state "All passives basically suck".
  
You have a problem sunshine that you need to address before you can make such a statement.

You you say your power amp gain is only 15db, this is half of what most amps are and is not passive friendly.

Who said 25kohm passive??? You should not have tried 25kohm passive, should have been 10kohm, as this is a match for any poweramp with industry standard of 47kohm input impedance or higher, and your source hopefully is low output impedance, not high impedance tube. 


So basically you suck it up and don't even go passive and complain about them or you fix your problems and maybe just maybe you may see the light.
That passives can be more transparent more dynamic and less coloured than any active pre with or without gain.

Cheers George