I have SQD-2000, 2010 & 2020 - they are all good little units (when correctly calibrated).
The main differences are -
SQD-2000 - total discrete components mounted on 2 A4 size single-sided pcbs (mounted one above the other)
SQD-2010 - custom ICs mounted on 1 A4 single-sided pcb
SQD-2020 - total discrete components mounted on 1 A4 double-sided pcb (riveted to join tracks from one side to the other)
SQD-2010 is in my opinion the best option (provided it is setup - leave the pots alone!!) it must have also been the most expensive to design, using the custom ICs - I have yet to find any information on the ICs. This unit does not normally "pump" like the other discrete units.
I have service manuals for all 3 + about 40 other quad manuals from Sony/JVC/Technics etc
The only things I am still searching for is a copy of service manual for my Sony SQE-2000 (SQ encoder) & ST-5555 (Tuner)
Here is my setup - I also have a very understanding lady also living in the house!!!
System 1 – mix of Sony & JVC & Toshiba & Eversonic
VT-700 (Tuner)
SEA-V7E (Equalizer)
SQD-2000 (Decoder)
4VN-880 (Amp)
KD-10 (Cassette)
HR-40 (4-Channel Headphones)
Eversonic (4 off) 50W 5-way
System 2 – mix of Sony & JVC & Philips
VT-700 (Tuner)
SEA-V7E (Equalizer) - front
SQD-2010 (Decoder)
SEA-V7E (Equalizer) - rear
4VN-880 (Amp)
5911 (4-Channel Remote)
5944 (4-Channel Headphones)
733K (CD)
System 3 – Sony & Technics & Magnat
ST-5150 (Tuner)
TA-1150 (Amp) – front
SQD-2020 (Decoder)
TA-1150 (Amp) – rear
D-5A (CD)
EAH-400 (4-Channel Headphones)
Magnat-6 Speakers (4 off) 160W 3-way
System 4 – PC system, to transfer LPs to CD
PC with sound blaster Live 5.1
Sound Forge software
Sound Laundry software
SL-10 (Turntable)
Terratec Phono Pre-Amp Studio
I also have the following other pieces in my collection.
Cosrad HS 4000 4-Channel Headphones
Hafler 101A Preamp
KD-10 Cassette – used for spare parts
JVC ECA-102 4-Channel Reverb.
Marantz ST-400 Tuner (modified by myself to include AM Stereo PCB)
Pioneer QD-240 CD-4 Demodulator
Quad 405mkII Amp
Sony SQA-100 Decoder (3)
Sony SQA-200 Decoder
Sony SQA-2030 Decoder
Sony SQD-1000 Decoder (2)
Sony SQD-2020 Decoder
Sony SQD-2050 Decoder (2)
Sony SQD-2070 Decoder
Sony SQE-2000 Encoder
Sony ST-80F Tuner (3)
Sony ST-88 Tuner (2)
Sony ST-2950 Tuner
Sony ST-5555 Tuner
Sony TA-88 Amp (3)
Sony TA-1055 Amp (3)
Sony TAH-10 Headphone adapter
Technics SH-400 CD-4 Demodulator
Technics SH-3433 Scope
Plus Service manuals for most of the above.
Plus approx 500 Quad LPs & about 50 Quad cds