Increasing the input load won't really reduce feedback, it will improve sensitivity.
There are 2 Blues Jr schematics, pre '01 and Mar '01 onwards. Many resistor cap numbers are completely different from one to the other. So if your amp is pre '01, then yes R2 is the input load.
However, these amps have a TON of gain so I'm not sure that it's the best basis for a harp amp. That said, you could try snipping the bright cap off the volume control (C2), if you can live without the fat switch try snipping C4, reduce the size of the feedback resistor (R27-say add 47K in parallel?), go a little larger at R13 (add 22k in series)?
Of course there are also preamp tube subs to try & rebiasing?
All these are easily reversible if things don't work out.