Before trying this revision, make sure your speakers/headphones are not too loud.
I have been looking at it for a long while, and I decided it couldn't be just "somewhat similar" to a Schroeder Reverberator.
The SPU2 designers would have actually used a working design for a reverberator, so chances are they implemented Schroeder's.
I haven't had the chance to test this code in spu2-x, but the results in the "Impulse Response Analyzer" tool I coded specifically for this, are positive.
Some tweaks might be needed, because I'm not sure if Neill got the IIR part right or not.