Dark problems with Plustek 7200i using sane & xsane

Am attempting to use a Plustek 7200i scanner connected via USB using 
xsane (0.999) frontend.

Versions:

    Linux Mint version: Linux Carbon 6.8.0-57-generic #59~22.04.1-Ubuntu
    SMP PREEMPT_DYNAMIC Wed Mar 19 17:07:41 UTC 2 x86_64 x86_64 x86_64
    GNU/Linux

    sane-backends: 1.2.1-debian; backend version 1.3.1

    'genesys:libusb:001:010' is a PLUSTEK OpticFilm 7200i flatbed scanner

Am attempting to scan 35mm slides, with xsane source set as 
"Transparency adapter" and medium as "Slide".

xsane detects the Plustek scanner just fine, and completes scans happily 
too.

However, with default / installation settings, the scanned images appear 
to be clear in resolution but are _much_ too dark.

Playing with xsane settings (brightness / gamma etc) mitigates this a 
little but not much. Behaviour is consistent regardless of resolution of 
scan.

Doesn't appear to be a simple hardware fault, as (the preview version 
of) Vuescan does not have the same issue.

I'd be grateful for any suggestions. Could it be a calibration issue??

Thanks!