Preamps built Into DACs

There are DACs and there are combo pre-amp/DACs.

One buys what one needs.

Not sure I see a problem there?

In English language bunch of different words, expressions may mean the same thing and yet the same word can have a tons of different meanings. These factors you should know in order to stay within our 3d without going beyond.

DAC with volume control

Preamp with digital input(s)

Full function preamp(usually applied to the ones with phono)

DAC-preamp

preamp-DAC

...will basically mean the same and share the same idea

Conserning your second paragraph, I believe you're tripping or indeed found yourself in multi-d space and have no idea how to get outahere. Saving money by removing DAC? or preamp? How about removing DAC from CD-player? I don't getitatall. I've always believed that one box will be cheaper that two boxes.
One preamp with DAC and phono will keep you from necessity buying both of separates...

I used to study the multi-d math, but kinda quit doin' that coz got tired understanding those theorems that are necessary for you not to get lost.
To continue studying multi-dimentional calculus I would probably have to smoke an ounce of MaryJohana per week.

Mitch - Beware replacing a good active pre with a volume control in a DAC. Most of them are substandard and will be a step down.

On the other hand, a few are excellent and will probably beat your active pre. Depends on the technology and implementation. It varies widely. I would read the reviews. If they say it beats good preamps, then its probably a good candidate.

Some are based on variable gain control of an active buffer stage. Others are based on resistive attenuation combined with a gain stage. Some are based on using an integrated chip to digitally control the gain of an analog stage. Two of them are based on controlling the reference voltage to the D/A conversion.

The last 2 generally deliver the best SQ, but again it depends on the implementation, the design details. Controlling the D/A reference voltage is the only one that actually reduces distortion as the volume is decreased.

You can also use purely digital volume control, but I dont recommend it. More than about 9 dB of attenuation will start to reduce sound quality.

Another option is to use a passive transformer based volume control (TVC). You basically cannot lose doing it this way and it beats pretty much all active preamps.

Steve N.
Empirical Audio

This is a good question, the ps audio perfectwave dac mk2 has a preamp section in it, wondering how good it could be