i would of answered this earlier but i had to head out real quick. by bad connections like the rca jacks in the back? also by not working do you mean not at all or just static/crackling? if it's scratchy simple burst of deoxit will fix up right away or just some electronic cleaners at local radioshack( 2 different kinds. one for contacts and is also a lubricate and other is a de-oxidizer. the de-oxidizer is the one to get). could try that first. have you also tried headphone directly to test?

for replacements you can get the nad fixed or just buy another one. if not. sansui CA preamps, kenwood C-X, and B&K are some to look at. yamaha preamps are great as well and are very quiet and do offer some nice features in the C-XX and CX lineup. the C-65 or C-70 be nice start if you want to try those.  perreaux are nice as well and like their power amps i think their preamps use mosfets as well. mosfets are considered improved ''tubes'' due to their very low distortion,lower then 0 ohm output impedance and ability to soft-clip like tubes. B&K and Adcom are known to use mosfets as well. yamaha only used mosfets in their bigger power amps. sansui and kenwood don't. that reminds me too... sony and hitachi use mosfets in lot of their gear so can check their preamp line-up from the 70's and 80's.

another thought could also always get a receiver or integrated amp to use as a preamp by removing the jumpers in the back. like the sansui 2000x can be had for very little money compared to it's big brother 5000x which can actually go for good amount depending on condition. even the 4000 can be found nice price and will give that warm clean sound but that depends cause they're pretty big so to you it might not be an option due to their size.

i would try searching locally so you can test them out personally if you can.

Don't mean to be a thread jacker, but maybe someone can give me a few suggestions as well.

I'm looking for a vintage receiver with a phono input and a good headphone out. I have a couple of Technics 1200s collecting dust, so I figured I'd use them for a little vinyl rig. Will use them with HD800 and LCD-2, so preferably something warm-ish as to not upset the Senns. The main recommendations that I get are Sansui 7070/8080/9090, Marantz 22xx, Pioneer SX-xxxx, but they normally go for like $400 in my area. If they're worth it, so be it. But if there are comparable receivers for cheaper, well then that means more money for vinyl.

Bonus if it can run a pair of Maggies since I'll eventually get a pair whenever I move out of this tiny apartment.

I don't know, isn't a preamp what is typically used in this situation?  It's pretty much like running a power amp connected to a pair of speakers. 

RD, if you just need a volume knob, why not just a passive RCA volume controller? It's just RCA in --> Volume Knob --> RCA out. I don't have any experience with it myself, but I've seen them for less than $50.

EDIT: something like this? http://www.amazon.com/Scosche-RLC-Remote-Level-Control/dp/B002EEP3WA/ref=sr_1_6?ie=UTF8

x2 on a passive.

passive is a great choice but it will probably will not give the warm sound you want but will offer everything else. lot of passive devices are made to be transparent as possible so looking for one for warm sound might be bit hard unless you can get a tube passive device or a tube dac. i mean those are active monitors so preamp will probably not be necessary unless want extra inputs/outputs or you monitors need a +4dbu gain at it's inputs. 

preamps are basically control amplifiers. you don't need one exactly to get sound from the power amp but depending on your source's voltage output your poweramp will be under-driven causing not only lack of gain but can attribute to lack of bass/sub-bass,dynamics and stereo separation. power amps need certain amount of voltage at their inputs to run at their full output and to be able to have dynamic headroom. lot of power amps need to operate atleast with a +4dbu gain which is 1.23v RMS. some amps even need as much a 1.7-2v at the inputs depending on it's purpose and how much power the amp has.

lot of balanced audio interfaces and mixers have a +4dbu gain for professional gear and more then capable of hooking things up like power amps if needed. consumer gear is -10dbv cause things like receivers and cd players need only mv to operate at their inputs. hooking a +4dbu interface/mixer to a receiver or any other consumer gear can cause it to clip like hell from over driving the inputs. there is some interfaces/preamps/mixers with a switch for -10dbv or +4dbu gain for consumer and professional gear. lot of preamps have a voltage output of 1.5v maximum at it's pre-outs(that's why lot of people also have good results driving very high impedance headphones from the pre-outs).