The trick with setting up a satellite-sub system properly is to always, always shoot for a splice frequency of 80Hz. Tom Holman determined this frequency back in the eighties when he was developing THX up at George Lucas' ranch. And the Dolby guys, who were (and still are) up in SF at the time agreed that 80Hz was a best splice frequency because most (mostly ported designs) 5.25" mid-woofer monitoring systems rolled off (-3dB) around 80Hz.
A well-designed 4" mid-woofer will roll-off around 100HZ -3dB, but a 3" does really well if it can get down to 120Hz @-3dB. A 2.25" Bose sat doesn't start getting on the flat low-end part of the frequency curve until around 150Hz but psycho-acoustically it is much "easier on the ears" to have a suck-out in the 80Hz to 150Hz region, which most people think of as being flatter sounding, than to have a phony bump which, when coupled with a poorly set-up sub, non-flat at the 80Hz splice-point, sounds like mud. )
The object of the whole 80Hz splice is to be able to obtain a Flat response at 80Hz, not one hyped with a bump via porting designed to make the satellite itself sound like it can put out more bass that it should be doing in the first place. So a trick to try in addition to running RABOS on the sub is to plug the port on a 5.25" or 6" satellite speaker at the same time.
About RABOS. Greatest invention that nobody ever used. Seriously. I run two Infinity subs, a CSW-10" and an Intermezzo 1.2 12", both of which have RABOS built-in. Both subs are also in room corners so I get a whole lot of free SPL gain which naturally comes in the form of a huge bass boost at ~70.5 Hz because my ceiling is 8 feet high and 8 feet is the half-wavelength (of 1130' /sec divided by 8' = 141.25 Hz). Without RABOS to get rid of the 70Hz hump(s) I'd have more than a 9dB boost at my listening position. And my Audyssey Sound Equalizer (ASE) is software-driven to only account for a maximum of 9dB when it does its magic. So my bass-room calibration procedure was to first spend about two hours tweaking the response of my two subs across the three-person listening positions of my sofa. Using the RABOS on each sub I was trying for the best flatness-balance of the two subs across the three sofa listening positions. I did this Before I ran Audyssey's calibration sequence on the ASE. So, by the time I did run the ASE calibration the huge 70Hz room peaks were already gone from the bass-to-midrange signal Audyssey's calibrated microphone would "hear". My left-right front sats are my (own-design) Beta 10s and my current center is an Infinity Modulus which was designed by Pedro Manrique who is now at M-Audio.
Regarding setting a 4 ohm or 6 ohm switch on the back of a receiver. Two reasons for using the 4 ohm or the 6 ohm. This can be a crap-shoot. First, at Infinity (and JBL and Harman and Revel) the design edict for at least the last 10 years as far is impedance is concerned is that no loudspeaker can dip below 4 ohms within it's passband, even as we designers rated our speaker's as "8-ohms", knowing that almost all of our designs dipped to just above or slightly below 4-ohms most times in the low bass region. Second, bear in mind that receiver manufacturers have to get their designs through UL before they sell them. And receiver manufacturers aren't (usually) also speaker manufacturers.
Having a 4-ohm switch on the back of a receiver may mean that to pass UL when using a 4-ohm resistor load the receiver's protection circuitry has been designed to go into protection sooner because of temperature rise (caused by increased current). Conversely, the 6-ohm position may typically correspond to the receiver being able to more easily pass UL before temperature rise causes the protection circuitry to kick in; that is, a speaker which dips to 4-ohms with the receiver switch set to 6-ohm may be able to play very slightly louder because it might have a tiny bit more headroom, more power kinda-sorta "in reserve" available before heat-tripped amplifier protection circuitry cuts in.
In practice it may be very, very difficult to tell what was the amp manufacturer's thinking when they put that switch on the back. But all manufacturers want to a) get their their products sold first, and for that reason b) a switch is put on the back so they can get the receiver through UL in the first place. Use full bandwidth pink noise such as can be found on the RABOS CD while switching between the 4 or 6 ohm position. If you can hear a difference in SPL pick the switch position which appears to play a bit louder. You may be letting the receiver's power a bit more breathing room as far as heat build-up is concerned before the protection circuitry kicks in.
As far as I've been able to determine this switch thing is a modern day patch made to protect receiver manufacturers so that they can keep their products sold and their reputations for building quality products in tact. The genesis for this switch, again, as far as I've been able to determine, goes back to the early eighties when I was Yamaha's first product manager and I would get on Yamaha Japan's case for not quoting their R-300,R-500, R-700, R-1000 and R-2000 receivers' power into the traditional 8-ohm load. Yamaha was trying to cut costs by abandoning discrete power transistors and going to VLSI's which couldn't run into a 4-ohm load for more than about 30 seconds. Conversely, these VLSI's couldn't put out rated power at 8-ohms so Yamaha chose to go to the 6-ohm rating at the same time they introduced their atrocious sounding VLSIs. Fortunately I got Yamaha to see the error of their ways and they went back to discrete transistor power amp designs after that R-series.
And what happened to the VLSI's? Yamaha Japan sold those VLSI's to the Koreans who were just getting into consumer audio and didn't any better at the time. Orphaned Yamaha VLSIs showed up in the next generation Sherwood receivers. Glad we're past that period in crummy sounding audio! Say what you will about bit-rate limited iPods. If asked to equivocate I would guess those Yamaha VLSi's of almost thirty years ago would sound like a ~32kB/sec music stream today.