There is no one length or impedance that is o.k. or not o.k. - it's not a 'go / no go' kinda thing. Any impedance source will still play music, etc..... Most systems are just fine and I am sure yours will work out - but since your cables are longer than typical just look for a source with a lowish output impedance - use 300 ohms or less as a guide. If you like the source and it is 350 ohms then go for it, etc. I think you'll find most sources have an impedance of under 600 ohms or so - the majority being 300 ohms or less. Chances are that your chosen source will fit the system fine.