You should be out doing 75 mile rides to prepare for this, over comparable terrain at the least. Within ten of these rides you'll know if there's something you need to replace on your bike. Go out and work your way up to 75-90 mile day rides and you'll quickly find out what equipment works and what doesn't.

Lazyass has it correct. These numbers agree with Jan Heine's 15% tire drop method. You can go as low as 45 psi front and 70 psi rear to achieve optimal comfort. I'd go with 50 front and 75 rear, just for a little fudge room.

If you try this, make sure to check your tire pressure more often; don't let it get too low. You're a lot closer to the pinch-flat zone at 50 psi than you are at 90 psi.

Keep messing with the tire pressures until you find what is most comfortable. It's probably lower than what you're riding now.

I've been going off that chart for a long time. With my bike I weigh @180. I go with 70 front and 110 rear, sometimes a tad less. My main ride is the aluminum Focus with 23c's, most of my roads are chipseal and it's smooth enough for me. And it's funny, the last time I ever had a pinch flat was back when I would jack the pressure up to 120 front and rear. Used to get them all the time at those pressures.

I only run about a 10 psi difference front to back. Sounds like I could go quite a bit lower in the front?

Thin-walled (read: expensive) steel is springy. Commodity 4130 used to make most budget steel frames is thick-walled and doesn't have any particular advantage in ride quality over aluminum. Roughly even odds that it will even be worse, based on choices made regarding frame design and spec.

All-City: will ride like a brick. Surly: definitely rides like a brick. Soma: may ride like a brick, but no personal experience to be certain; however, it is in the "rides like a brick" price range. Gunnar would be pretty good. No idea on the others.

That doesn't mean these bikes are uncomfortable, just that how comfortable they are won't have very much to do with what they're made of. I've had a Surly LHT and a Surly Cross-Check, and both of them were comfortable. But it wasn't because they were springy. They were, as I said, bricks. But with the right tires, they were very pleasant to ride.

As a general rule, going to a different frame material isn't a very reliable way to improve comfort.

I think you should get a wooden bicycle. Everybody knows that wood has a great strength to weight ratio and is flexible too! Home - Renovo Hardwood Bicycles

This I can not agree more.

My experience is the opposite. My Colnago (Columbus Gilco) which would be expensive is far from springy and my cheap Miele (chromoly) is springy. It's in the shape of the tubing that affect the ride characteristics. Colnago star shaped, Miele round.

All steel is springy, but what matters is whether you can feel that spring. Like you said, the gauge of the tubing makes the most difference.

High-end heat treated steel can be thin-walled because it's strong. That means it will bend to and fro but spring back to its original position.

"Normal" 4130 CroMo steel has to be drawn a bit thicker because it's not quite as strong. That means it may flex a bit, but you may not feel it as much.

Cheapo steel is high-tensile, and it has to be damn thick. Usually it is non-butted. Think old cheap MTB frames: heavy but indestructible because of the thick-walled tubes.

I don't know the gauge of tubing used in Surly and All-City bikes. I understand the LHT rides like a brick unloaded, but feels pretty good with a bunch of weight on it. I really hope that Surly uses thinner tubing in its roadie models like the Pacer.

Soma has some options in heat-treated Tange Prestige, for pretty reasonable prices:
Smoothie ($430), ES ($430), Stanyan ($710), and San Marcos ($930). I've seen the 1G San Marcos frames (pearly light blue) on clearance for around $750 online.

I also recommend that the OP keep his eye out for an 80s or 90s frame made from high-end heat treated steel. Since the OP is pretty light, he probably won't feel the comfy springiness on a heavier gauge frame. In the pre-carbon era, there were lots of choices in heat-treated steel and they now can be bought for relatively cheap. You can fit modern mechs on most any high end 90s steel frame.

I have two '97 bikes with thin-wall heat treated steel frames, one in Reynolds 753 and one in French Excelle Eco tubing. Each gives a better ride than any carbon or aluminum bike I've ridden.
I also have an '88 bike in Tange Prestige, but it must have thick-walled tubes because it's inflexible, even under my 230#. That bike is a rigid MTB, eventually destined to become a loaded tourer.

Not sure of your timetable for this ride but frame material is not as important as bike fit as long as the frame builder and design is good. Commuting 10 miles on the saddle you have will feel very different at 75 miles for your 2 day 150 mile ride. Start riding longer distance on what you have to see how your body is reacting...adjust as necessary. If there are many hills within the 150 miles I would start doing some hill repeats during the week to add miles. Have fun.....