Is this how a Subwoofer Crossover is supposed to work?

I wish they came in 5 or 7 db attenuation. -10 starts taking the problem in the other direction LOL.

10dB attenuation is not a problem at all, it give the volume control on the subwoofer more freedom for adjustment! 

I'm not understanding, why not use the gain on the sub to attenuate the output?

I bought two Starke SW12 subwoofers that I installed. So far I’m not particularly happy with them. They are way too loud even with the volume set almost to off.

@ erik_squires, beside the +6dB of xlr connection I think the subwoofer amp has higher gain than the tube amp that OP set to. Another solution is set the volume control on the tube amp higher, but OP have to lower the volume control on the DAC it might degrade the sound quality.

Pinwa, welcome to the contorted world of subwoofers. Audiokinesis  is very versed in this subject. I have been involved with subwoofers on a semi professional basis since 1978. 
You are quite correct, those are very bad crossover curves for what they were indicated to be. The peak at 70 Hz and the dip just above 30 Hz are room modes. Make believe they are not there. Duke is correct in that it is possible for voice coil induction to create this effect but I somehow do not think this is the problem. I've looked at a lot of woofer specs and I have never seen one roll off this low. My inclination would be to send them back as they do not meet spec. With the Moabs I would want a crossover somewhere around 80 Hz, the 3dB down point. These are obviously not going to make it up there. It is unusual to see a sub not going up. The problem is usually the sib going down! 
If you are a handy person you might want to take a look at the Sub kits offered by Dayton labs. They are really quite excellent and better performing than a lot of commercial subs.
Using a high pass filter on the main speakers is very important for two reasons. It increase the headroom of the system and lower distortion on several fronts. If the supplied cross over does not have one you can supply your own first order crossover just by putting the right value capacitor in series with the + input of you amp. The math is readily available on line. All you need to know are the crossover frequency you want to use and the input impedance of your amps. I did this for years.
Ideally what you want to do with two subs is place them between the Moabs right up against the wall. You place all the speakers and subs on a radius from the listening position so that they are all the same distance from your ears. Then as long as none of your equipment shifts phase you will be good to go. You might consider adding more subs in the future if you don't get enough thud. 
The best subwoofer management systems are digital. You might want to look at my system page and you can see this in action measurements and all. My unit is no longer available but there are similar units on the market By DEQX, Anthem, Trinnov and now McIntosh. The problem with computer run systems is that you have to have and control 4 channels of DAC simultaneously. Using two separate DACs creates issues.
Please feel free to contact me if you have issues.

Mike

Ahhh.  Of course, an XLR to RCA adapter generally does the same thing, but it also unbalances the line.