Usual suspects I guess maxcatch Barrio has a new stock of lines in and they are good quality worth a look..

 

Depends on the grain rating for the rod.
Some are single handed rated and some double handed. So saying #6 doesn't help too much.
What sort of rod taper is it? Fast and tippy or progressive/semi-para/regressive?
The former, a Scandi is more appropriate, the latter either Scandi or Skagit will do.
Medium length spey lines will also be applicable but probably stick to heads of length under 50' or it's much harder work for a shorter rod of 12'.
If it's a 6 double rated then I guess something like a 385 grain Scandi or 400 Skagit.
I'd use a running line (either mono based or a pvc coated one) and loop on different heads. Much more economical.
Aiflo make a compact Scandi and Compact Skagit which are quite reasonable and available on discount sites cheaply.

 

Also look on the Rio spey line recommendations charts on their site, it has typical grain ratings for lots of different makes of rod if you don't know.
The A rating is for an experienced caster, the B for a new spey caster (normally heavier with more grains)

 

Orvis does or did a line for this rod .i have one and like it a lot

 

Orvis recommend the Mission Skagit 420gr, Mission Shooting Line .031" for that rod. So anything similar is likely to work.

https://www.orvis.co.uk/store/produ...ore/product.aspx?pf_id=2S76&dir_id=441&group_id=442&cat_id=9055&subcat_id=30614

Rumour has it that you can use a trout line that's 3 weights heavier on a small spey rod so a #9 trout line might be fun to try. But you wouldn't get the shooting head action, more like a short-bodied traditional spey.

 

Ive used airflow 40+ and maxcatch saltwater outbounds on switch rods, eg the outbound WF10 goes well on a greys 7/8 switch and 8/9, the 40+ #9 on an oracle 7/8 and an outbound wf7 (18g) on a bloke 6wt (trout) switch, although none have the delicacy of say a Barrio switch.

With a new switch rod i just start messing with various lines, including those much heavier and when i get an amount out of the tip that feels good i measure it and weigh it and go from there.

Check out the forum on Barrio's site, someone has provabbly asked the same question on there about your specific rod.