I know this is an old thread, but I've just tested another Spec1 (original, untouched) and yes, it exhibits the same poor HF response. In fact, I'd call The Spec1 a flawed design by Pioneer. Remember they rated this Spec 1 as 10Hz-70kHz +0/-0.5dB and sold it as TOTL! (see specs)
With 150mV in the line (aux) input (rated input for 2.0v output), the only positions where the response is flat to 20KHz are "16,18,20" which correspond to between 12:30 and 2:00pm on the volume dial. This 150mV input however, results in only 174mV out, hardly the position where a typical power amp would be driven to full power. Consider Pioneer rated the unit at 150mV sensitivity for its rated output. Trouble is, at rated output and 150mV in, its high end response is dreadful, rolling off above 13kHz.
At full volume, the response is still rolled off by approximately 0.4dB at 20kHz and continues to dive until at 30KHz it is almost 3dB down.
At detent '22' (12o'clock high), we have a nice 2v out (unity gain for 2.0V in), but we are still 0.4dB down at 20kHz.
With 2v in (typical of modern CD/line sources at 0dB) there is only one position where a flat response can be obtained. (see below)
With 2 volt in (aux) we have the following:
Position 33, 0.576V out and -0.3dB at 20kHz
Position 37, ruler flat (!)
Position 30, -0.35dB at 20kHz
Position 42, +0.8dB at 20kHz (yes it is now rising at HF)
These are all with a 50k ohm load as Pioneer specified, although the output impedance is rated around 600 ohms.
By comparison, an inexpensive Yamaha CX-600 preamplifier from the 1980s gave me a response out to well above 50KHz before any roll-off was evident and a Sony TAE77es preamplifier was flat from 0.5Hz to well above 100kHz (it's rated to 300kHz).
I will say however, the tone control action (if anyone ever uses them) is the best I have seen with gorgeous curves not extending into places where they shouldn't. The low end response is not flat, but pretty good considering the massive number of caps the signal goes through.
None of that helps however, the preamplifier is not worthy of its reputation which is most likely based on build quality rather than actual performance. I've had three of them now and measured two- but they both exhibited the same issues.