You don't take the RIAA equalization into account.
In a nutshell on a vinyl recording the bass frequencies are damped by up to 20db. The reason for this is among other things that strong bass-signals would cause the needle to jump out of the grove.
During playback the exact reverse filering is done to the signal (e.g. the bass is boosted by 20db).
In a normal playback situation the two filters cancel out and you hear the original recording (well - kinda.. the curves are always a bit off).
If you however play back the record at a wrong speed the filters don't match up anymore and you get a filtering response that's all over the place. This is part of the fat sound that you'll hear when you slow down the turn-table. In the extreme case you'll get up to 40db boost on the lowest frequencies that way.
To get a better scratch effect you have to apply the high pass filter to the raw .wav signal, and after playback (at whatever speed you want) you apply the inverse filter.
Make sure that you have a way to deal with *very* high signal levels. Inside the turn table the preamplifier will go into soft saturation.
You can simulate this if you just route the signal through a saturation function like tanh.
This will simulate a overdriven transistor pair almost perfectly. Adjust the amount of saturation to taste and you'll get a much better scratch effect.
Codes for RIAA emphases should be out there. Otherwise it's okay to just build one by concatening three simple lowpass/highpass filters in series.