Upgrading for multiple tracks - What interface

http://www.sweetwater.com...annel_Preamps/low2high

Since you're already accustomed to Focusrite products, it would make sense to just go with one of the 8i6's multi-channel siblings. They're inexpensive and pretty decent interfaces. I currently use a Saffire Pro 40, which was half the price of the MOTU 828Mk2 it replaced but provides nearly identical functionality.

Hey hold the phone !!   A search for Sapphire 8i6 takes me to info about the Scarlett 8i6, Sapphires never had numbers like that. So if you have the Scarlett 8i6 you already own a 8 input interface...Are you just not setting it up right? 

As pointed out above, the saffire unit has 2 mic preamps, 2 separate line inputs and digital spdif in/outs.  If you have a mixer or outboard preamps you are good to go for two more channels.  If you get something like the Tascam uh-7000 you can use its superior preamps and conversion over the spdif digital in - and out for montitoring.  That gives you 6 inputs at once.  The uh7000 is $400.
 
That would be a cheap way to improve inputs and quality.  Otherwise, how much are you thinking of spending?  I saw a bigger Focusrite for $500, tho I don't think you can aggregate it w/ your unit.  Lynx has a new 4 x 4 pcie unit coming out.  If it is priced like the old unit it will be $1000+, but it includes 2 digital in/outs so you could use the saffire.  Or just buy Burl. 
 
A price range would help.

 
The Mackie is a only a stereo interface @ $299 that's what you'll get. The 8 channel to DAW version is $650.  http://www.mackie.com/products/dlseries/
All those budget USB mixers drivers are not very good for Sonar.  They are not intended to be used as an interface. It's for stereo board mixes and playback. 
  
You are not really engaging in this thread very well ( 3 weeks to reply to your own questions) so I'll be brief. I am more then willing to sort this mystery out for you if you could just get back in a timely manner please.
First I think you own the interface shown below. If this is correct you already have 2 XLR inputs, 2 TRS inputs and SPDIF for a total of 6. If it is not, could you supply a link to the product page please. 
 

I have it sorted out now ---you are sure correct about my not engaging well. Appreciate your efforts to help me out. That is the interface I own. 

OK, well you still only really have 4 inputs unless you have a SPDIF mixer or pre amp on hand. And you were after 6 or 8, so you might have to upgrade to the 18i8.
Like you were originally saying the advertising can be misleading with some brands.
 
 
Like the 18i8, unless you use the digital and Adat connections you only have 8 analog inputs. So 18 inputs is a bit misleading for most people. 
 
And for some folks if you don't have a dedicated pre amp or a little mixer, the line inputs often found on the back,  are useless for microphones.
Example- Your 6 input 8i6 is only good for 2 mikes without using additional equipment. 
Buy the time you pay for the additional equipment you are often better off buying the right interface with the exact connections you need. The more wires and connections, the more noise in your signal path. 
Many seem to make this mistake when purchasing an interface. 
 
I have the Scarlett 6i6 which is more or less just like yours. I can use all 6 inputs together.   I have a Yamaha 01V digital mixer so I use the SPDIF inputs primarily for my collection of hardware synths and some mikes. I have dozens of things plugged into the 01v so I never have to re patch most instruments. I also use it for the headphone mix. 
I have a TC BH 250 Bass pre amp into one line in #3 and A Joe Meek threeQ pre amp into line in #4 that is my main vocal input now. 
I actually don't use the front panel XLR inputs unless I have more than on person in the studio. 
So I leave those open as they are nice and fast to set up with just about anything  Mike or DI.
I never liked them for vocals.