Two Rotel preamp sound different

There you go different pieces of equipment sound different even from the same manufacturer. Glad your new preamp sounds better than the old one.

If they are different models, even from the same brand, it is highly likely they sound different. That's obvious. Unless I am missing something here?

All my preamps uave a different tone. 
this is on 2ndary basement system 

 B&k pro10 mc is smooth with no harshness.  Adds depth to midrange and is a nuthair bright. 

sanders “preamp” is the cleanest nothing added straight wire w gain ive ever used

Very honest with recordings and bad is bad and great is great. amazing features, there is “zero”  change in sound unless i set tone controls  

onlyo P-308  adds an overall  very natural low frequency tone to entire audio spectrum during playback  it adds some lows to recordings which need it  and does not cut off highs with playback  midrange s more crunchy w p-308  

When i switched to the b&k in mid late 90’s from onkyo tx-890 used as preamp for the m-504 and matching tuner/eq/sl-1200mkii tt. Jvc tape deck/wollensack 8 track. 
 

tape and 8 trk have been cleaned wrapped and in storage in other room. 
eq in closet. Havent really used in years. 
i do need new rca intcts one of these days. The beldens are nice as are the gronberg ts premium. 
 

when i bought and swapped the b&k pro10 for tfhe P-308 i was “wow” i like that. 
sounded beefier.  I use the Sanders now, i like the very natural and uncolored sound with many recordings. 

"If they are different models, even from the same brand, it is highly likely they sound different. That's obvious. Unless I am missing something here?"

If this were true you would only buy the entry level of every offering out there. Yes, you are missing something.

Many years ago I owned both of these preamps.  The 970 was not offensive but it's bigger brother was a richer, fuller more controlled sound even via the SE outputs.