Simplistic NJFET RIAA

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1st pic left channel, 2nd pic right channel, both always feed 1st the 1kHz from signal generator & after connected the scope at the output of Low MC pre-pre
 

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Hi
where can I look for the source of the sibilance that I have? The rest of the components of my system can be ruled out as there is no sibilance when I play cds, for xmpl. I have Grado black that needs only 47k load according to manufacturer, no loading cap. I tried various loading caps from 10p to 250p but no luck. Instrumental music sounds fine, no extra hi range or it is less noticeable so it is no problem.But on some vocal inclusive vynils it really gets anoying.I'm sure this sibilance is not product of the preamp, yet I don't know where to look for the cure any more. Apart from the preamp, what else can introduce this nasty distortion?
As far as I understand 100n cap in the preamp is not part of the RIAA, it is just DC block?
Thank you
 
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15k load= some improvement, but not much.
VTA looks good.
No dirt.
Grado wear- about 800hrs.
Most of my vinyls are brand new.
Rs tried from 28-100ohm - no change- apart from level.
JFETs are matched well, I've done it several times before, no problems.
I'll try the "psu last cap" thing one of these days.
I'll also change all the psu caps to audio grade (now I'm using Panasonic FC and Nichicon PW for the 16V cap on the RIAA)
Any other ideas?