Many thanks to everyone for the advice and information.  As I noted above, my gigantic problem was solved by rebooting my Focusrite.  btw, there's a cute little MIDI analysis freeware called MIDI-OX that helped to point me to the Focusrite.  I recommend it.
 
My use of Sonar is pretty exclusively for rough demos of arrangements.  8.5 has done fine for me, but if it's not certified for use with Win10 it's probably time to upgrade, even if I don't really need the add-ons - I'll probably end up using some of them if they're there. 
 
As to that, should I first try adding RAM to my existing computer, or is my processor speed an issue now?  The machine is only about 4-5 years old, but apparently that's the modern equivalent of driving a '78 Dodge.
If I were to backgrade to Win7 that would mean getting a second computer for everything else (I sense that many of you have done that), and resigning myself to no updates or support for my old programs.  
 
I do still get Finale 2014.5 crashing fairly often since the update to Win10.  The MakeMusic customer service guy has been really helpful in trying to solve the problem, but so far it persists.  I imagine a clean re-install of Win10 would be my next step, but what a headache. 
 
Any processors you all can recommend?  Any that I should stay away from?
 
Dan

As to that, should I first try adding RAM to my existing computer, or is my processor speed an issue now?

 
I'd say both. Your existing motherboard may also limit the amount & type of RAM you could add.
 
So as a bare minimum, you're looking at a new MOBO, RAM, CPU, PSU
This to me says new computer.
 
If you're reasonably tech savvy consider building one yourself. You could salvage your existing case, hard & optical drives, but this will depend on what you currently have.
 
Many users have built their own machines and collectively we're a pretty knowledgeable bunch and can guide you through the process.
 
Just something to consider.

Thanks for the reply.  I'm going to get a quote from my computer repair guys down the street, whom I like.  It would be fun to build my own machine - I'm geeky enough - but as always, my "musician" time needs to take precedence.  Also, my hunch is that buying those elements separately might not save me any money over buying a machine put together by people who can buy the parts in quantity.  Not true?

On a different note, my biggest issue since Win10 took over my machine is that Finale is crashing constantly.  As I mentioned earlier, the tech support guy has walked me through a bunch of attempted fixes, but nothing is helping.  I un/re-installed it, changed drivers, bypassed the Focusrite, created a zillion event and crashlog files, even let him wander around my machine remotely for awhile.  His latest suggestion is that I create a new admin account on my computer.  My current account is admin, but he thinks this might make a difference; something about permissions.  He swears that Finale 2014.5 is fully compatible with Windows 10, but the proof is in the pudding IMO; something is causing the crashes.  Being an old-school guy, I keep suspecting an interrupt conflict, though I honestly don't even know if those are a thing anymore.
 
A possibly-related problem is that Win10 stops being able to play YouTube videos.  I can fix the problem by putting the machine to sleep and waking it up; then - voila - they can play again.  Since Finale only crashes sporadically - sometimes it'll go for half an hour before crashing, sometimes only a few minutes - it's hard to correlate the two.
 
I might solve this problem by obtaining a new machine.  But the nerd in me wants to know why the crashes are happening . . . 
 

my hunch is that buying those elements separately might not save me any money over buying a machine put together by people who can buy the parts in quantity.  Not true?

 
Not true. 
 
You could spend a grand on components and build a monstro machine that could easily cost you twice that amount if built professionally.
 
Are your computer guys completely up to speed with what's required when building a machine dedicated to audio production? I would guess the answer is no.
 
Have a ponder and get back to us. Establish a budget for yourself and most importantly, give us an idea of your intended use. Will it just be you recording & overdubbing or will you be recording full bands?

I do arrangements for HS show choirs mostly.  I have to create rough demos of the arrangements for a small ensemble (usually guitar, bass, drums, keys, synth, and horn section) using a MIDI keyboard to create all tracks (not at all studio quality).  I don't even put actual vocals on the recordings, but just a dorky-sounding synth "voice" sound to approximate them.  They pay me for the arrangements; the demos are just to give them (and their combo) an idea.  
 
I do several dozen of these per year, as well as an occasional chart for band or a take-down of a song.  I'm also a musical theater composer, and would like any prospective new setup to be able to record live vocals (again just for decent demos); probably just one at a time, since I'm not in a studio.  For that, I imagine the Focusrite can do what I need with regard to main/monitor functions, and I have a multi headphone box.
 
In the past, the current setup, with Finale, Sonar, an I/O unit and decent computer have been sufficient for the choir demos.  Since the Win10 update, Finale crashes constantly, but I'm working through that, since I don't have a choice.
 
I would love some input as to what I should make sure my computer guys include in my new system; especially if there's particular hardware one should use, or stay away from.
 
Thanks.