Low level inputs are always better to run from my understanding. You get less noise interference from low level inputs. If you have low level inputs available, you should by some rca splitter cables (y adapters) and run low level inputs to each amp.

I believe the Legacy LA-290 (4x100w) does not have onboard x-over, and the deck has only 4 channel fullrange.. so an outboard two way (lo-pass/hi-pass) x-over is needed for the subwoofer and the 6x9s. and just use one set of rca from the deck.

I'm finding your first post is a bit confusing.....

or if you're running another amp, just use an amp with onboard sub x-over to drive the sub. and use the LA-290 for the rest of the speakers. and use front out from the deck for the fullrange(6x9, etc) and rear out to the sub amp.

Do you have only the 1pr of 6x9 speakers? (the Legacy amp is four channel)

I think what above poster is saying is to run your rca output to a crossover like this one

SoundStorm SX200 Crossovers

then run the output from the low pass filter to your sub amp and the output from the high pass filter to your speaker amp.

errrr ignore this please

I have a Kicker 2 way electronic crossover i'm using to split between my subs and 6x9's. Works good.
I'll be re-installing a new HU and won't need it anymore if you'd like to buy it.

Another thing you could do is buy an amp for the subs that comes with a remote for sub level control. A lot of high end sub amps come with built in crossovers and pass thru RCA's for feeding other amps.
Here is one example:
http://www.pioneerelectronics.ca/pna/product/detail/0,,32171715_226727547_226727554,00.html

Ok, to clear things up.

I will be getting my Kenwood KDC-205 Deck, in the mail soon.
I want to connect 2 amps to it.
1st amp being my Legacy LA-290, 400w, 4 channel. To power my 6x9s.
Then another amp, haven't chose yet, to power my 2 subs.

Now, I think my deck, only has 2 audio RCA outputs. So those 2 would go straight to my sub amp.

BUT, what I am stuck on is this.
People keep telling me to run the Low Level Inputs to my amp, instead of the High Level Inputs.
But, the only way I could make this work, is to run the High Level Inputs. Because the 2 Low RCA outputs from my deck, to go to my other amp, will be taken up.

Will the High Level Inputs to my LA290 sound the same as the Lows ?
Or, what could I do to avoid using the High Level Inputs, and using the lows, on only 2 RCA outputs from the deck.

Your deck only has 1 rca output as stated here.

Run your rcas to a 2 way crossover like this *click*. From your cross over (hpf part) run to your amp for your 6x9's. Run from your subwoofer part of the crossover to your sub amp. This way you can have control of your sub frequencies externally as well as the cutoff frequencies of your 6x9's.

Make everything easier to use a 4 Channel Line Output Converter, to convert my output lines from my deck, into RCAs, So I can run those 4 RCA wires to my LA 290, then the other 2 RCA outputs on the deck, to my sub amp ?
This sounds good no ?

The Legacy amp still has no crossover so that won't help much. A 2 way crossover will let you filter out the low frequencies that your speakers shouldn't be playing. It will also split your single RCA's into 2 sets, one for each amp.

Split my Single RCAs to run to each amp wont help, because my LA 290 has 4 RCA input holes, ( 2 sets, 4 channels )

I think I may try with the converter, and if the low freq's are very noticeable, then I will get a crossover.

Edit. This is the crossover I would need http://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?ProductID=15881