I want the game to present stat blocks that offer a challenge right out of the box.
A few thoughts:
1. The game is not built for your specific party, it is built at a base level. So if you have a group that is tactically savy and/or play optimized characters to eek out every last bit of power, well then you will have to adjust from the baseline. I have one group that would, as you have it set up, have ROLFSTOMPED that encounter. I play in another group that would have more than likely TPK'd to it. WOTC can't build the game to fit every situation, that is why there is a DM. If you don't want to spend the time to tweak your game, I would find a new group of newbs to DM for, things will likely work out better for you.
2. The game is built and balanced on a bunch of assumptions. First and foremost, encounters per day. But also Magic item availability, ability stats, and a player experience level. Any of these things being off can and will have dramatic effects. I don't know your groups stats, they sound fairly versed in character building and at least moderately tactical. But the big thing is, 1 encounter. If you try to run single encounters where the players can blow their load every time, the monsters will always be pathetically weak. You likely don't want to force the baseline number of encounters the game is built on, and that is fine, I don't either. But if you are not going to follow the encounters per day, then you will need to spend the time tweaking, customizing, and up-scaling the encounters you do have. You can't have it both ways. Either you follow the guidelines and play the game as intended, or you don't and spent time tweaking things to account for it. Complaining about weak monsters when you aren't running the game correctly is a bit silly. It would be like buying a car that is supposed to get 20 mpg then adding a supercharger and complaining that they lied cause you are only getting 15. Run that same fight again as the last encounter of a 6+ encounter day when your PCs are low on resources and then come back and tell us how it goes.
3. Flamestrike had some valid points, not about the OAs, but in general. If you have a group that is tactically savvy and optimized and you go into it throwing crappy damage spells and cantrips at them in the first round instead of battlefield control spells and tossing out the big stuff, they are gonna wipe the floor with your encounters every time.
Now with all that said, I have a group this is a Barbarian, shadow monk, wizard and a tag along pally ran by the barbarian player. All optimized, with a decent amount of magic items, and VERY tactically savvy. I actually threw a very similar encounter at them at nearly the exact same level. Difference being I customized all the bad guys hand picking their spells. I chose the bad guy's special abilities with my group in mind. I started out of the box concentrating on battlefield control and nullifying as many of them as I could, and I started them with their buffs up. My players won, as I knew they would, but they blew most of their resources, the pally went down at one point, and there was still a few touch and go moments. The big thing is, I am not DM'ing for a baseline group or using the base encounter guideline, so I know I have to put some time in when designing my encounters because of it and I do exactly that. And I understand that the encounter difficulty scale doesn't apply for my game. My players always win, which is what I want, but most of the time it's tough and close and they really have to use their heads.