Winter Cycling - Best Bottoms Lightweight Flexible Baselayer or Long Underwear or Tights

So I have a nice pair of windbreaker pants from REI, the Headwind pants:

http://www.rei.com/product/724365

These are just fine down into the low 40s. 30s and 20s they are not so comfortable though. I'd like to get a lightweight bottom baselayer or perhaps a long underwear or tights that I could wear underneath the Headwind pants.

I bought a pair of cheap base layer bottoms from Target that were awful because they were not very flexible. Relatively speaking biking requires a large range of motion with your legs, compared with say jogging or walking or swimming so I really need something flexible.

On the other end of the price spectrum, I have awesome merino wool tops but I think merino wool would be too warm as bottoms given that my legs are pumping when riding.

A lot of long underwear is cotton and mid or expedition weight so it would either soak up sweat or be too bulky and is not appropriate when layering under other cycling clothes. I think I need something that is lightweight since I'm keeping relatively warm from pedaling and already have the wind breaker layer on top.

So is there any bottom (links greatly appreciated) out there in your experience that is lightweight and flexible to wear under a windbreaker type pant?

I wear Nashbar tights ... and Walmart knickers on days when the temp dips down below freezing.

And I replace the Walmart knickers with sweatpants on days when the temp dips way below freezing.

A pair of polypro (http://www.nashbar.com/profile.cfm?category=6000137&subcategory=60001218&brand=&sku=10272&storetype=&estoreid=&pagename=Shop%20by%20Subcat%3A%20Accessories%20-%20Base%20Layer) or merino wool (http://www.performancebike.com/shop/profile.cfm?sku=17472) long johns under my patrol pants serves me well between 0 and 40F.

If you can't live without a chamois, a pair of bib tights/knickers might be more to your liking.

I've got the same pants and really like them. Since they breath well, I just wear duofold longjohns underneath. Very comfortable down to around zero.
But as chipcom posts, polypro or wool would probably be preferable to duofold.

All of the above!

I wear regular biking tights over bike shorts for commuting down into the 30's.

For colder weather I'll pull looser pants over the biking shorts (I have several pair of generic black exercise/running/rock climbing pants of different types - like Pearl Izumi Tokyo Tights).

I have knee warmers and leg warmers that I transition to as it gets colder and then eventually my chamoisless tights. I wore the knee warmers underneath the tights the other night and it increased warmth considerably. Likewise the leg warmers could be worn at colder temperatures. These three articles of clothing provide for a lot of flexibility across a broad range of temperatures. The DeFeet Knee warmers are very inexpensive and most importantly, stay in place. Bellweather leg warmers are relatively inexpensive also.

Thanks Chipcom. I did not know that Performance had those merino wool bottoms, for the great price of $20 too, my merino wool tops cost about 2.5 times that! Funny, I was at Performance yesterday night and did not see those.

I can definitely do without the chamois since I have a Brooks and do not ride as much in the winter.

Performance stores don't carry a lot of the stuff they offer online, as I have discovered after wasting a trip to their store here a few times. In fact, I've often caught the store with higher prices than online...but to their credit, when I point it out they verify and give me the online price.

I get my merino wool sweaters from either Target (Merona brand I think) or the thrift store...but I gotta go online to find merino wool baselayer tops, bottoms and undies.

I wear a thin polypro base under similar pants for down to about 20F, recently picked up another nice set at Costco for $19.99, fantastic. Colder than 20 I go for wool long johns, then various weights of tights as a base layer.

The best piece of winter cycling gear I have is a pair of Craft layer one long john's with a wind front - front panel. Made a huge difference in my commute once the temps are below 15 F.