Basically, i'm wondering if there would be a huge difference in ride quality, going from an 80s bianchi steel frame to say, a more modern tig welded steel bianchi frame?
No, there will not be. There will be a dig difference in marketing cost to the manufacturer and that will be passed to you, but you have already spent that money.
If I had to guess on the differences in ride quality of modern steel vs. modern steel of the 80's I would bet the average road rider would not know the difference provided the geometry was the same.
Anyone who states otherwise must be finishing a career as a pro-rider and has the experience on both, or they have bought into the marketing hype on "new" welding techniques.
I know others don't agree, but that is my thought on the subject. I just see bike makers having to come up with an edge over the others to sell bikes. Bottom line is you pay for wieght, modern steel frames weigh less, sometimes as much as a pound compared to quality bikes of the past.
Regards,
ehenz