Sorry, this might be a bit difficult to explain.

My interface is a Tascam US-1641. On the back of the unit are Speakers OUT L and R. Obviously my speakers are plugged in accordingly.
However, in Sonar there is no 'speaker out' option for the master track and it is actually sending out of line out 1 and line out 2, but this is actually the speaker outs.
 
Now, this has never been a problem but I'm expanding out board a little and have three or four units that I send via line out on the back of the tascam.

The tascam has two speaker outs and four line outs.
Sonar is technically only letting me send out of line out 3 and 4 because line out 1 and 2 are being 'used' by the speakers. Does this make sense? I'm new to the world of outboard so there is every chance I'm hooking it up wrongly so any tips greatly appreciate.d

Thanks.
 

SONAR will never know what hardware is really connected to your PC and never communicates directly to the ports on the devices. SONAR communicates with the driver(s) you installed for the hardware. SONAR will only show the inputs and outputs supplied by those drivers.

I haven't checked the Tascam documentation, but I'm guessing Speaker L-R and Line 1-2 outs are parallel signal paths providing different voltage/impedance for the two purposes. If SONAR is only showing two pairs of output channels presented by the driver, that's almost certainly the case.
 
 

Hi. I would believe that brundlefly is spot on with this one. It has been that way with the audio interfaces I have owned.
Best of luck.

brundlefly
I haven't checked the Tascam documentation, but I'm guessing Speaker L-R and Line 1-2 outs are parallel signal paths providing different voltage/impedance for the two purposes. If SONAR is only showing two pairs of output channels presented by the driver, that's almost certainly the case.
 
 

That makes perfect sense mate. Thanks to you and everyone. Understanding is half the battle. I'm guessing this is where a patch bay / signal router of some kind may be considered for the job?

What's the end goal?

There isn't one really. I suppose I'd like to be able to use the outboard I have with minimum effort. I could just plug the units in and out as I need them I suppose.

Behringer do an inexpensive balanced rack mount patchbay. I've a couple of the older unbalanced versions and they're actually pretty good and very reliable.

The Tascam us 1641 ( and us1800) are 16 in and 4 out. 
The monitor outputs comes from Sonars Master out  (default is 1/2 out)  and is the best place to attach your studio monitors. They can then be controlled by the front panel monitor level as well as the blend mix control ( computer / input) 
The 1/2 and 3/4 are not controlled by the front panel so can me used to send to a mixer etc. 
 
It is noted that the SPDIF output shares with output 3/4 in the Tascam control panel so either one or the other can be used but not both. So no matter what 4 outputs just like they say when they advertise it as a 16x4. 
 
Make sure you download the latest drivers which were released last December. Big improvments where made and give new life to these units which suffered from pour drivers in the past. 

Cactus Music
The Tascam us 1641 ( and us1800) are 16 in and 4 out. 
The monitor outputs comes from Sonars Master out  (default is 1/2 out)  and is the best place to attach your studio monitors. They can then be controlled by the front panel monitor level as well as the blend mix control ( computer / input) 
The 1/2 and 3/4 are not controlled by the front panel so can me used to send to a mixer etc. 
 
It is noted that the SPDIF output shares with output 3/4 in the Tascam control panel so either one or the other can be used but not both. So no matter what 4 outputs just like they say when they advertise it as a 16x4. 
 
Make sure you download the latest drivers which were released last December. Big improvments where made and give new life to these units which suffered from pour drivers in the past. 

Thanks for the explanation man appreciate it. I got the latest drivers but it has caused untold problems on windows 8.1 so I've rolled back the drivers a few version and it is a bit more stable. I think (sadly) the interface might be the next real studio upgrade.