Dynacord 5010B solid-state bass amp, lots of hiss in preamp

[alex.veuthey]

Hello fellow repairers, first post here :-)

It looks like this is an active forum, and I can't figure this out on my own, so here it goes!

I have an old (mid-late 1970's it seems, the serial doesn't tell me anything) Dynacord 5010B, which is pretty nice, except for an annoying high-frequency hiss... Here's a bit of info (in German) http://www.el-me-se.de/old/pdf_files/5010B.pdf

Here's what I know about this particular amp:
- power amp section transistors were replaced in the 90's after being blown by an active bass in the "high" input...
- from what I've looked on the rest of the circuit, no components were replaced

And details about the issue:
- there's a mid-high freq hiss with significant impact (it's much louder than the typical amp background noise/hum/badly-shielded-pickups thing, although it wouldn't be heard with continuous playing I think)
- the hiss is present even without an instrument plugged in (either with no cable inserted, or with gain at 0, which completely cuts the input)
- also present on the fuzz channel, if I remember correctly, but can't check it and its effect on noise right now, because I don't have a footswitch at home...
- the gain control doesn't really affect it (it's slightly louder in the middle 90% of the control range, though)
- the volume control cuts the noise at 0 and makes it really loud at maximum, which tells me it's coming from the preamp section
- I haven't checked the signal path, but I'm assuming the EQ controls are in between the gain and volume control
- regarding EQ: bass doesn't affect, mid makes the hiss more mid-y, treble makes it more treble-y, presence makes it more , this indicates that the noise is created before the EQ controls

My main problems are:
- can't find the schematics online, except for paid options on shady websites, which I don't trust. Has anyone paid & downloaded a good quality diagram from websites like manuals-in-pdf.com ? e.g. https://www.manuals-in-pdf.com/download-5010B-DYNACORD-p-514029.html
- not really familiar with that kind of repair,  never used an oscilloscope (though I can borrow one from a friend)
- electronic shops in my region are pretty much guaranteed to refuse looking into that, due to the age and value of the amp (seems quite low), also don't want to pay for that and more interested in repairing it myself for experience

Are my assumptions correct?
Could anyone point me in a more educated direction?
Is there something I can check with a multimeter (or two)?
Would replacing all the preamp transistors (there are only 6 I think) get rid of the problem, or could it come from other components? Then I'd need to find appropriate replacements, but that will come in time.

Thanks for any help!