4 channel amp in 3 channel mode, 1 RCA or 2 + Y adapter?

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SivaNevets
SivaNevets Posts: 303
edited July 2010 in Car Audio & Electronics
MB Quart Onyx 4.60

Mannual here, page 6
http://www.maxxsonics.net/manuals/mbquart/pdfs/MBQuartOnyxAmplifierManual2010.pdf

I want run this amp in 3 channel mode(1/2 to front components, 3/4 to sub), I would like to use 1 set RCA cable instead of addtional RCA and Y adapter, the mannual says the amp will sum the signal and do the low pass for me. Which way would be better? My Hu does have sub preout though. if both method work the same why should i bother with the Y adapter? What will be the difference?
Front: Martinlogan ESL
Center: Martinlogan Stage
Rear: Martinlogan Motion 4
Sub: Martinlogan Grotto-I
Receiver: Pioneer Elite SC 1523K

PC 2 Channel: Polk LSI7
Headset: Grado RS2 + Grado RA1 amp
Mic: Neumann KMS605

Car Audio
2002 MB C240 Sedan
MM6501 components
MM840 sub
MB Quart Onyx 4.60 (1/2 to components, 3/4 bridged to sub)
Pioneer 8200BT HU