The closest to the old Invincible settings is +2 levels, one hero, yes to bosses (and maybe yes to AVs). What you're seeing is not really the new difficulty settings, but actual changes in how likely bosses were to appear at all. Even if we didn't have the new settings we'd likely all be seeing more bosses among more NPC factions at a smaller number teammates.
Blue
American Steele: 50 BS/Inv
Nightfall: 50 DDD
Sable Slayer: 50 DM/Rgn
Fortune's Shadow: 50 Dark/Psi
WinterStrike: 47 Ice/Dev
Quantum Well: 43 Inv/EM
Twilit Destiny: 43 MA/DA
Red
Shadowslip: 50 DDC
Final Rest: 50 MA/Rgn
Abyssal Frost: 50 Ice/Dark
Golden Ember: 50 SM/FA
Welcome back to the game, Shiny_Victory! 
I think you'll also find some good information located here on the wiki.
Leader of Legion of Valor/Fallen Legion (Victory server)
http://legionofvalor.guildportal.com / http://fallenlegion.guildportal.com
StainedGlassScarlet - L50 Spines/Inv Scrapper | Badges: 1,396
Avatar detail taken from full-size piece by Douglas Shuler here
I asked about how to make the new settings as close to the old ones as possible, and this was the general consensus:
Heroic/Villainous: +0/x1
Tenacious/Malicious: +0/x2
Rugged/Vicious: +1/x1
Unyielding/Ruthless: +1/x2
Invincible/Relentless: +2/x1
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And honestly if you set to */x1 with bosses on, you will not see many bosses while soloing. I have several mid lvl chars set on +2/x1, bosses on, and it is very rare that i even get three bosses in a single mish. Oddly +2/x2, bosses on, seems to spawn a lot of bosses (about every third mob).
Thanks for the info, +2 level and bosses on seems to have done the trick... at least until I get off my butt and buff up my guy with a bunch of those nifty inventions so I can raise it more.
I'm an old player who just recently hopped back into the game and I've got some questions about the new difficulty options you can choose. They seem really cool but I'm not really a fan of how the boss situation works with soloing from what I've seen thus far; I remember that back in the day when I soloed missions at the Invincible level I got a good mix of minions and lieutenants with a boss at the end. With the new options however it seems like I have to either have no bosses or a mission that is filled with way too many of them. Is there a certain difficulty set-up that would give me something close to the aforementioned enemy spread = groups of minions and lieutenants with just one or two bosses thrown in?