I found I had very little light output from my lights as well. The problem with mine was very slightly attributable to the lenses, as they were pitted and a bit 'foggy' from age and wear.

But the majority of my problem was the reflective coating on the reflector inside the light housing. The coating is very thin, and when it doesn't reflect, you don't get any light output. A new headlight assembly fixed the problem.

The isn't really any way to refurbish the reflectors, as the metalization process used is quite complex/expensive and unless you live close to a place that will do it for you, there's really no way to get it done for a reasonable price.

New lights range anywhere from ~$85 up to $600 or even more if you're really looking to get hosed.

The crappy aftermarket ones are actually relatively good if you're looking to not spend any money. The replacement projector/elipsoid lights look like an awesome replacement option if you're looking to get a bit more performance at a bit higher cost.

Cheapest lights I've found online thus far:
http://www.lampsandlenses.com/epcgetspecials.epc?cookieID=2V80W1GJR2V80W1P5N&drillid=4&level3=HEADLIGHT


FCP:
http://www.fcpgroton.com/category-exec/category_id/24/nm/Headlights/sub_top_menu_item/by_make-by_model-by_year/by_make/78/by_model/1464/by_year/48

IPD sells the projectors:
http://www.ipdusa.com/Volvo-S70V70-1998-2000/Lighting-&-Lenses/Projector-headlights-&-Turn-signals/p-128-371-4861/
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1998 V70 AWD->FWD Turbo 200k+