The Soundsplinter RL-i 8" looks beefy..

 

4 x cdt ef-8cf ...

 

John... just build one to what you need... that way you decide what you get... might take a few tries... but you will get what you want

 

It seems like the winner to me. I have heard a variety of 8" subs, and the closest competitor to the RL-i was the 8W7...at almost twice the cost!

 

There is also the tc-1000 from tcsounds. It takes more power then the rli but is very similar to the rli.

 

Not many 8's "excel" below 50Hz like you want.

As you mentioned though, the 8W7 is a great choice, very low Fs and high Q driver makes it play very deep for an 8".
However it also requires a box larger than some 10's and is more expensive than alot of pairs of quality 12's, lol.

As "thewavecaraudio" mentioned... the CDT Audio EF-8 is a great subwoofer, exceptional transient response and very linear response... however for good output below 50Hz, I'd strongly advise running it ported... this sub will hold together very well and sound much more accurate ported than most subs do sealed. Also box size will still be very small, equal to most sealed boxes for other model subs.
Pricing on these is also great for the performance they offer... so if money is at all an issue, these should be your choice.

Another great option is the Kicker Solobaric 8" L7... now don't start flaming yet, lol... kickers are really not known for their SQ I know... however these little beasts are the black sheep of the solobaric line... very accurate and a great sounding sub that can handle gobs of power.
Best of all they'll play great right down into high 20Hz!!!

Similar to the 8W7 being a high Q design with a fairly low Fs for an 8" driver.

They do have an advantage in cone area compared to all other models being square... however their footprint still remains that of being an 8" driver, which seems to be the issue with you.