Stacking

I would never stack speakers on top of subs!! Way too many vibrations going both ways. If space is limited put the subs next to the speakers or stuffed in the corner of the room. Put them next to a couch or chair and stick a lamp and a plant or some coffee table books on the subs. Blocky end tables. 

@danmar123 Wouldn't smaller subs remedy the need for stacking?

From what I've been gathering is, I would need a larger sub than the Tuba 24. People have been suggesting the table Tuba. With that unit, I haven't any room upfront. Right now my LaScalas are 9.5 center. If I add the bass units up-front, that would make the LaScala s 7' on center. That would be my best bet ,I guess. 

Right now, I have not run all HT wires under the floor, so my subs are stacked in the corner. It works, but it's not optimal. DSP helped, but they need to be moved. 

What did help was putting isolation pucks under both subs, decouple the subs from each other and the floor. 

Not wise. 

The subwoofer generates strong low-frequency energy and vibration, which can transfer to the speaker and create additional resonances that interact with the speaker’s own natural resonance. Additionally, the La Scala is 40 inches tall, with the horn located around 33–34 inches—right at typical ear level when seated. Placing it on top of a subwoofer would raise the tweeter and upper midrange horn above the ideal listening height, potentially affecting the speaker’s imaging and soundstage precision.

Over time, the continuous vibration can compromise the structural integrity of the speaker cabinet. By the time cracks appear—either internally or externally—it may already be too late to prevent damage.

Thank you for the reminder. This makes me realize that next time I’m considering a used pair of speakers, I should ask the seller whether they’ve been placed on top of a subwoofer for an extended period.

Lastly, the ideal placement for a subwoofer is not necessarily directly beneath or near the speakers. Experimenting with different positions is key to seamlessly integrating the sub into your system. Limited space should not be an excuse — it may even be possible to place the subwoofer along a side wall.