Vintage speaker upgrade suggestions?

Forget about SACD!

Please don't take this the wrong way, but you will not get hi-end sound from your buying methods. But if you find that you enjoy the sound, then you are lucky, because you will not catch the "audiophile addiction", which can be quite expensive.

First, the Marantz is about 25 years old, and should be checked and most likely refurbished (capacitors) which would not be worth the money.

Second, all of the speakers you've listed are were "college dorm specials"...about 20+ years ago! They're so old, plan on replacing the drivers. Again, not worth it

Third, what you have assembled may be good for "head-banger" rock and roll, but for the type of music that you like...forget about it.

You cannot assemble a decent mid-fi, let alone entry level hi-end system, with your "flea market" buying attitude. You can definitely get lucky, and occasionally find some great vintage equipment at garage sales or swap meets, but what you have doesn't qualify! Do some research, and read the posts at Agon regularly. Just because it's old doesn't make it "vintage".

I disagree (respectfully) with Fatparrot, in part, and I agree, again in part. I would say that the A70's and the Marantz qualify as vintage equipment and possibly the Missions as well. I think the SR2000 would drive the A70's quite effectively as they are an easy load. I still own a pair so I speak from experience. I also think they would do a good job with your musical preferences. As to whether they or the Missions could replace your Bose, I feel that is a matter of personal preference. I certainly feel the A70's are superior but it is really your opinion that matters.
Fatparrot is correct that your equipment may need some service. However this may not be all that expensive. I would recommend that you check out audiokarma.org where you can get advice and information from a group who specialize in vintage equipment. Lots of Boston and Marantz fans there and good, helpful people too.
I'll also agree that SACD is probably not the way to go unless you plan on going to a multichannel system in the future. As long as you stay with 2-channel systems you really won't be able to take advantage of the SACD format.
Anyway I hope you enjoy your equipment. I think you'll find it can give you a lot of musical satisfaction. Good luck.

Thanks for the help. I'm going to check out the A70's soon. Also, thanks for the referral to Audiokarma, it seems to be a good place to lurk around.

In defense of myself, the speakers and receiver had been checked out and repaired already at time of purchase. Second of all, as a college student living in a small dorm room, I'm not headed for a pair of Nautilus's and McIntosh gear any year soon. I'm Just trying to get the best I can for the most I can afford. A little advice in that direction certainly wouldn't hurt.